I really liked the boss battle at the end of DOA4 against Alpha 152. It was good and challenging and really gave your reflexes a work out. It was something you really had to concentrate on, because if you let your guard down for a second or missed a counter it was all over, which made it all the more satisfying when you landed a finishing dragon kick to the face with Jan Lee.

A good boss battle: The final colossus in Shadow of the Colossus (der) because it would RAPE you if you didn't even figure out how to get close enough to him.

Bad boss battle: Probably a toss up between the final Ganondorf fight in Twilight Princess (it was insanely easy, but pretty cool looking) and Kain Versus the Elder God in Legacy of Kain: Defiance.

Hmm, to find some that haven't been mentioned yet...

GOOD: Taking down General Song in Mercenaries was pretty sweet. It's not an actual *boss fight*, but you're battling through his missile bunker on a time limit, stopping (or not) the nuclear countdown, shooting down air reinforcements and chasing the bugger down on foot. Then, instead of killing him, you can (at least as Mattias Nilsson) give him a knee to the groin, a headbutt to the face, kick him to the ground and handcuff him before calling in an airlift to haul him off to all the comforts Guantanamo (or wherever) can offer... Pretty satisfactory, actually.

BAD: No one has said it, so I will. Graven, from Conan on the PS3/X360. I mean, first of all the guy looks like a toybox outcast, the bastard child of Stretch Armstrong and Silly Putty. Then he has a bunch of incredibly cheap attacks and counters, dealing MASSIVE damage to you while your attacks do next to none. When he is at zero health, you need to hit the right buttons in order, in a very short time, or he knocks you down and regains half his health. This is all acceptable. Having to do it THREE TIMES, with him getting full health each time while you don't, is not. Oh, did I mention that between those rounds you have to avoid his rapidly spinning tentacles WHILE battling minions AND trying to turn a wheel? You do.
OK, you can regain A LITTLE health after each tentacle round, if you can kill the last 4 minions without being hit by them or Graven. Sounds fair, doesn't it?:P
For crying out loud, on a much earlier stage I fought an ELEPHANT DEMON, with flaming eyes and everything! THAT was a good fight. The end boss is a not so much a battle as an excercise in frustration. Bad end to a pretty good game.

(On the subject of Conan, I have to say DON'T BUY "AGE OF CONAN". I loved it at first but Funcom's mishandling of the title, lack of updates and customer service coupled with the innumerable bugs and lack of content made me hate it in the end. May it rest in peace, as quickly as possible:)

Good One: POP 2 - Dahaka - I love how he speaks backwards
Bad One: Bioshock - Atlas/Fontaine - the weakest boss ever

Worst... Hate to say it but the last boss in Half Life 1. I mean, what the fuck was all that about? A giant fetus? Did not good at all and sadly the only real tactic seemed to be shoot it for ages and jump in the teleport it shoots at you when you are getting fucked.

Best... Ripto in Sypro 2 was pretty fun (Yes, I friggin' love spyro and it was the worst day of my life when I realised they had pussied him up so much for the PS2) I also loved the Lich in Guild Wars Prophesies but I did that mission with a friend the other day after not having done it for about 2 years and guess what? They pussied him up. He's still bit of a badass but not how I remember him.

the whole sequence from devil may cry, form the evil pope portion to the giant living sparda statue was good, as was glados and ganon, but my favorite would be xemnas from kh2, that was just really fun.

worst is fontaine form bioshock, after a fantastic art deco game were treated to a terribly unoriginal metamorphosis boss who is just a large muscular guy with the powers of all the other splicers put together, it was easy, poorly designed and unoriginal, and after a stellar game it was a moot point to end on some doom reject.

The Bandit:

Any boss who is invulnerable except for a short, randomized section is bad. I believe General RAAM was like this, because I can't recall any sure tactic to make the Krylll fly away from him.

Shoot him with a torque bow and those Kryll bugger right off. Pretty awesome =7D

Bad: Yu Yevon - FFX - Any boss that can kill itself is pathetic
Good: Rhapthorne - DQ8 - Mostly because I was severely underleveled, and the music is epic

Good end boss: Nyx [Persona 3]
A great final boss fight for a turn based RPG. Instead of having some monster with a titanical amount of HP, they had a varied, interesting fight that didn't just become a curing fest.

Bad end boss:
The boss I just described in most turn based RPGs. I guess Final Fantasy is guilty of this mainly.

There's a lot of other disappointing end bosses, like Mario bosses, Resident Evil 4's final boss, No More Heroes' final boss, etc.

I'll have to agree with Lewrawen on the bad boss, because you can just cast zombie on him and he will kill himself.

Ganon in OoT has to be one of my favorites.

good end boss: Vergil from DMC3, all three fights are stellar and really test your skills, especially on DMD mode

Bad: the final fight aganist Sephiroth, what's the point of having a boss fight if it's literally impossible to lose?!?!

Let's see...

Good bosses:
Chrono Trigger - I don't know why, but beating down Lavos was always, and still is, immensely satisfying to me. Maybe it's because he has a neat gimmick but doesn't throw it in your face.
Prince of Persia: Two Thrones - Say what you will but that fight was cool. It even saw fit to make use of the Prince's climbing ability more than his swordfighting ability, and let's all face it: we played the game to climb, not to stab.
God of War series - Every goddamn boss you fight in this game, and it's sequel, are not only fun as heck, but you feel damn awesome after doing it too.

Bad Bosses:
Castlevania SotN - Is it just me, or is Dracula getting more and more retarded with each iteration?
Assassin's Creed - Deserves a second (fourth) mention.

Oh yeah, like whitemidget mentioned, Shadow of the Colossus. The whole game was one big series of epic boss fights, and they pulled it off with styyyyle.

good: Mothership from Earth Defence Force 2017, easy but pretty damn epic

bad: Sturm from Advance Wars, It's not even funny how hard that guy was

Good: The Boss from MGS3. That battle is awesome, all thanks to the intricate combinations of camouflage, food tactics (that sounds stupid, but if you've played the game you know what I mean) and the setting - A field of white flowers while THAT music plays in the background.

Bad: The Vizier from Prince of Persia: The Sands of Time. The battle's annoying enough thanks to the terrible combat, and the fact that the Vizier constantly hides behind unbreakable shields and clones only adds to my annoyance.

Honourable mention goes the battle against the End in MGS3 since he might not be a final boss battle, but that fight is absolutley superlative.

good end boss... portal... the game was awesome, the boss humorous and challenging

bad end boss... ninja gaiden II, it wasnt actually the final boss that annoyed me... it was more the fact that the final boss was 5 bosses in a row... if i hadn;t been saving healing thingys all through the game it wud've been impossible

Good boss: Azel from GodHand. He was actually the second to last boss but anytime fighting him was a mix of challenge, frustration, and fun. Actually getting caught in his rapid fire pummels and then having to mash the buttons to outlast him was highly satisfying each time.

Bad boss: Levanthan from Odin Sphere. Honestly I thought this was the most boring and annoying boss fight as all it consisted of was climbing up a long dragon avoiding obstacles so you could get to his head and use Shadow Ally three times to kill him. The fact that you had to do it with Gwendolyn made it even more annoying due to her ability to glide instead of double jump. Honestly, compared to the other characters' final fights with King Gallon, Onyx, and the Cauldron respectively it was really disappointing.

Good end boss: Metroid prime from the game Metroid Prime. Took everything you earned and learned throughout the game to beat it.

Bad end boss: Bowser in Mario galaxy. Just swat stuff at him and dodge his easy to doge attacks.

Good: Kingdom Hearts 1&2 not very difficult but certainly fun and epic. Also all the different fights in them made for a good change of pace. It didn't do any one thing incredibly well but it was a good mix which I enjoyed.

Bad: Xenosaga 2. Not only are you reduced to using one character which kills any thought possible in an RPG, the music is terrible, and the boss is someone you have already fought before. Oh yeah and its not a fight either. He just stands there yapping while you shoot him.

Honorable Mention: Xenosaga 3. You fight the will of the universe who is piloting a giant robot powered by god's relics. Except it's way to cool to just be ONE giant robot, it's the pieces of a bunch of giant robots it's torn apart glued back together to create a REALLY giant robot. Also the music is so well done it could make taking the trash out seem epic. However despite all this awesomeness it fails to be a good fight simply because it is stupidly easy. So much potential wasted.

One of my favorite boss fights is O2 from Kirby 64. That boss has to be one of the hardest bosses in all of the Kirby games.

Good End Boss: Faceless Man- MOTB.

Bad End Boss: Sovereign- Mass Effect. Too easy.

I was hoping no one else would mention FF7! Haha! In all the games I have played, I have never kicked up such a fuss over a boss!! Although God Of War - Ares was pretty sick!!

Yeah that whole Sephiroth battle at the end pretty much kills it!!

Worst boss battles might have to be every Tekken game!!! I don't dislike the games, I just think the "Boss" fights are so anti-climatic!!

Good: Saren from Mass Effect. The fight itself was easy, but they way it was done was just awesome to me. (I have no idea why though)

Bad: Bowser, I hate that damn asshole, how many times do I have to kill him for him to stay dead....

Good: Ugh-Zan III from Serious Sam. Here's how they should have done Mehnruns Dagon. This guy was 100+ stories high, and all you had were your puny weapons to knock him over. And right as you thought you had him, he'd heal right back up to full. The only way to beat him was to use your head and pay attention to the giant flying saucer. Still, I almost crapped my pants the first time I saw him and realized I had to fight him.

Horribly, unforgivably awful: All of the bosses from Mechassault, ESPECIALLY Mechassault 2. Mechwarrior 2-4 was my favorite series as a kid, and I liked it because of it's heavy grounding in reality (as much as a 30 story, 100 ton robot can be grounded in reality). There wasn't any fancy jumping or sword fighting, it was huge walking tanks blasting each other to bits. Then came Mechassault. They took the deep customization and heavy realism and threw it out the window. Instead, we get a giant spider boss and half of an Atlas. I almost threw my Xbox out the window.

the final boss in link to the past... come one a couple of silver arrows and it's over... that's ..cheap man
one of the best boss battles I recall, The one in Dawn of sorrow it looked so awesome.
Most RE bosses are very well designed and very well balanced. And ofcourse the ones in the devil may cry games... capcom just knows how to make bossses

Good end boss: GLaDOS for obvious reasons.

Bad end boss: The guy at the end of Condemned 2. Seriously. It was annoying, broken, and above all, cliche.

Good: Final Nightmare (Legend of Zelda: Link's Awakening (GBA)) First appearing as the shadow beneath your feet as you drop into the final chamber this boss informs you how all of your current actions or more or less going to destroy the world you're in. He then darts from you, shapeshifting into various forms, each with their own way to be defeated (Knowledge of Link to the Past helps here). When you deliver the final killing blow, his last words, "This island is going to disappear... Our world is going to disappear... our world... our...world..." is rather chilling, and made me a bit reluctant to ascend the stairs to meet the Wind Fish. The music is fairly epic for a Game Boy game, too. There's a good video on Youtube when I searched for "Link's Awakening final boss"

Bad: I don't know what I hated worse, the final fight, or the ending to Thousand Arms (PSX) I'm not even going to get into it. Just generic and thrown together all around.

Good end boss: Liquid in MGS4. Great fighting system instead of just hunt and shoot with the normal 3rd person Metal Gear shooting system like most of the bosses. Metal Gear Solid 4 di a good job of mixing things while keeping things challenging. I remember having to retry this fight on normal difficulty. I'm fearing my Big Boss run. O.O
Overall, a good, fun, difficult bossfight.

Bad: Glados from portal. As great as the game was, I feel that they could have made up for the lack of gameplay with this bossfight, but just didnt deliver. I think it should have been harder and lasted longer.(could hav done w/o the timer)
They just made the bossfight short and...routine.
It was still fun, but disappointing at the same time.

Good end boss: Jack of Blades - Fable/Fable TLC
He wasn't the hardest boss but his fights were atleast fun.

Bad end boss: Fontaine - Bioshock
You take 1/4 of his health before you fight him. His attack pattern is predictable and repeats far too often. He is amazingly weak for someone pumped full of Adam, the splicers are harder than him. He uses fire, you use ice. He use ice, you use electricity. He uses electricity, you use fire. Or use the shotgun, he'll drop long before you run out of ammo with anything. I was nice seeing him get stabbed to death. Which leads me to ask: If he could bring you to near death in one blow why did he wait until he was almost dead to use that move? If I was him I would have crushed Jack straight away.

I'm not entirely suprised by the ammount of Atlas/Fontaine hate on here, but I still stand by my convictions that it delivered. Different strokes for different folks, I suppose.

A few other entries:

Good Boss:
- Kefka (Final Fantasy III / 6)
- Flying Polyps (Call of Cthulhu: Dark Corners of the Earth)
- The Master / The Lieutenant (Fallout 1)
- Frank Horrigan (Fallout 2)
- Saren (Mass Effect)
- Diablo, Baal, Mephisto (Diablo 2)
- Clan Mech Batallion (Mechwarrior 2: Mercenaries)

Bad Boss:
- Alien Squid + Huge Alien Blob (Crysis)
- Cyberdemon (Doom 3)
- Strogg Core/CPU/whatever (Quake 4)
- Tartarus (Halo 2)
- 343 Guilty Spark (Halo 3)
- Alma (Fear)
- Interplay (They killed Black Isle.....)

Oh, speaking of Black Isle - Irenicus in Baldurs Gate 2 was a MAJOR disappointment after such an awesome game. All you have to do is run out of sight and bomb the place he stands with cloudkills and other poisonous AOEs. He dies, and then he pulls you along to hell BECAUSE YOU ARE TOO WOUNDED TO RESIST after the fight...! Come on, no hit points lost and me and all my comrades are near death? Bad scripting, at least the hell fight was more of a challenge.

for me:

Good:
Dr. M (was it?) From Sly 3. He's an insane, super intelligent monkey that's been trying to get your family's jewels for about 10 years or so. I like the build up though, every character has a role that is different (though Dimitri's part was annoying) and then going through the huge gauntlet and fighting him in the end was nice.
Also the giant sphere O' death from Okami, you have to use all of your powers to get him, and he has a LOT of health.

Bad:
The Vizier from Sands of Time, it just wasn't climactic really, and I remember at the end, you had to hit him ONCE and poof, game over.
And Al'Mualim (sp?) was a pushover, do a couple counters with the hidden blade and yey, no more bad assassins.

good boss: sabaoth...Doom 3

bad boss: the eyeball thing at the end of halo 3

I just remembered another bad boss fight, had fallen out of my mind because I'm trying to forget that game - "GUN". The rest of the game is insanely easy, small and short, and then comes Lance Henriksen as the final baddie and is so difficult you want to break your controller, console, TV screen and sanity. It involves shooting dynamite out of the air, and blowing up geysers at the EXACT moment he is passing near them. Standard boss fare, but there are just so many factors in making it hard (ranging from bad controls, bad camera and bad collision detection to bad guy health) that I just felt like crying. He died at last, though. Only took fifty-seven attempts or so, and for comparison I might add that I beat King Bohan in "Heavenly Sword" on my first try...

I'll make up for it with another "nice" one. The last battle in Stranglehold is pretty good for an adrenaline rush, but then again, so is the entire game:)

Bad bosses: every boss fight in Rise to Honor (yeah i got suckered into buying this like a lot of PS2 owners :( )

Good Bosses: Most of the Meta Gear Solid bosses..except for The Fury, and Fatman.