I don't know if I'd call him a good or bad boss but sin from ffx is certinly a challenging one. It's frustrating trying to beat him but the satisfaction when you do is unmatched.

OverlordSteve:

Bad end boss: Al'Mualim, Assassin's Creed.
Urgh. I beat the entire thing with the counter. The only challenging part is that he keeps teleporting all over the damn battlefield.

Hehe, I beat him by using throwing knives after he teleported the first time and called me blind. I found him standing down in the courtyard and unloaded 15 knives into his chest, then I ran in and managed to get a high-profile assassination with the hidden blade. What disappointed me more was the "ending" to the game. I won't spoil anything, but people who've finished the game will know what I mean.

exocel:

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.

Unfortunately I have to agree. A bad boss in a bad game isn't as much of a cock slap as in a good game.

Best boss:
Hard choice but I would have to say in Crysis. On that note, just upgraded and can now run Crysis on high settings. It's actually pretty good if you can run it.

Crysis is like a friendly racist, if you're a low end pc then he hates you, if you're a high end pc however, he's great to you. It makes you feel a bit uncomfertable though because you've got some friends who are low end pc's and you dont like knowing he likes you for superficial reasons and is also a bastard.
What I'm trying to strain out is that if you can run it on high, Crysis is pretty damn good, if you cant, F&^% OFF.

jad4400:
For a dissapointing end boss fight I would have to say it was Dagon in the Elder Scrolls 4 Oblivion. You don't actually get to fight him, he's a giant 4 armed freky demon thing thats 10 stories tall. At first I was like ohh Sh*t this is going to be hard, but it will be interesting how I beat him. Then Martin turns into a dragon and POOF, Dagon is gone/dead. Seriously what the hell was the point of going on all those quests to stop this big evil thing and in the end not fight it (I will admit that most of the quests revolved aroung geting the amulet of king which Martin used to become the dragon so th the quests made sense in the end), but still could I have like fought Dagon while he is in a mortal form, then once I beat him he goes into an imortal form then Martin goes dragon and kills him, that would have not been so bad, at lest I would then get an end boss fight.

For a good end boss fight I would have to say Gongora in Lost Odyssey considering he was IMORTAL and had CRAPLOAD OF POWER and the ending was pretty good (kinda sad that Seth sacraficed heself to get rid of him though)

I know what you mean!! I ran past him into the building, that was it!

Ashur/Marduk from sacrifice, good boss, broke all the rules the game had laid out for you, making the fight very intresting.

Bad Boss: Dagoth Ur, he was such a pussy, i killed him in about 6 seconds.

GOOD: Lavos from Chrono Trigger. I love the fact you don't have to wait till the very end to fight him. You can take him on whenever you want really so the difficulty changes.

BAD: Another vote for FFX's 'Yu Yevon' - (spoilers) I liked the idea of having to beat Jecth and then kill off your summons first, but to make you invincible for the last boss? That's just a bit of a let down if you ask me. I was expecting it to merge all my summons into a giant monstrosity for one last big battle, then the credits rolled! :(

Good Boss: The 16th Colossus from Shadow of the Colossus, damn that fight was one big andrenaline rush also, anything wearing a tower for a dress deserves respect.

Bad Boss: Yu Yevon from FFX, one hit kill (well, was for me anyway) and you have auto-regen from the start. just a terrible idea.
I mean it worked for Sephiroth at the end of FFVII but Yu Yevon just didn't have the same kind of pressence as good ol' Sephy.

Good end boss: Van, Tales of the Abyss.

Other than the optional bosses like Nebilim, even if you're well leveled he's probably the hardest boss in the game. While his first form is rather easy (and the only thing to look out for is if he pulls an Imperial Slaughter out of his arse without going into Overlimit and Celestial Elegy, which I think hits everyone regardless of position), his second form on the other hand is an upgrade. Fortunately he has no Celestial Elegy and Ancient Requiem is probably the same thing as Imperial Slaughter or hell, radiant Howl, he still actually knows what he's doing. He doesn't JUST go for the healers or just one character, he actually tries to get your melees with stuff like Lightning Tiger blade while throwing other spells like Negative Gate and Ground Dasher at the healers and mages. (This includes Natalia since she can heal as well) Although you can be pretty cheap and find stun-locking patterns these often require perfect timing.

Bad end boss:

I have several pretty good examples of two end bosses.

1) Schwartz, Tales of Legendia

Even at freaking level 81 she nearly killed me even though I beat her on my first try on the normal difficulty. Maybe I just had a bad party with me or something, I was trying to exploit her weakness to Sea without falling to all of her melee attacks. She resisted nearly every single attempt to spell-cancel and would manage to stun-lock my guys to death. But I was also just as cheap to her as she was to me - Senel was hitting her for over 10,000 damage with his melee attacks and throwing her resulted in nearly 32,000 damage, and if every Maelstrom or Tidal Wave resulted in success then I could have freaking pwned her.

2) Darth Traya, Knights of the Old Republic 2, and Arch Angel.
This is the prime example of when the developers were rushed and just HAD to make a final boss that could deliver a challenge. Arch Angel was absolutely trivialized by how s/he didn't throw out near-instant-cast spells like Belias and whatever the heck they named Queklain and Hashmalum in Lion War on top of already shitty character balance. (Moreso in Lion War when Meliadoul's attacks could work on monsters) But even without Meliadoul and Orlandeau, Arch Angel's pathetic. I've actually hacked the game and found there are several attacks that could really hurt that s/he NEVER USES!

Now back to Darth Traya...She's hampered mostly by exploiting the lightsabers' horrible pathing ability, how they can mysteriously be poisoned, and how you're not really rewarded that much, just an ending that was obviously rushed. Still not quite as memorable as Malak, and it would have been a lot more memorable if you could have had Atris become Darth Traya and the two final bosses didn't just follow the same strategy.

All my favourite good bosses have been said; Gary Oak, GLaDOS, Fontaine, Yami, Ganondorf, they were all great.

And now, the bad. Sit back everybody, this is going to be a while.

Ganondorf in Twilight Princess. I was really pumped up for this fight, especially since the OoT and WW fights were amazing. I knew he was going to be the final boss after Zant. We all knew it. I walk up to his throne, and he says his first line to me, "Welcome to my castle." I'm quivering with anticipation, this is the moment I've been waiting for. Instead of fighting him, I'm fighting a possessed Zelda, and I have to reflect her magic blasts back at her. Okay, I've seen this before, but this is just a precursor, right? After this I get to fight him for real, and he turns into his beast form. Actually, this is the only part of the fight I really enjoyed, so I can't rant much about it. So after I defeat him, Midna goes super-sayain on him, while I get me and Zelda out of there. My next disappointment was that we didn't actually get to see them duke it out, next I know I'm in the field, with Zelda, and Ganondorf shows up with Midna's mask, and thus starts the horseback battle. This was very annoying, Zelda nearly always missed with her arrows, and it took me forever to get past it. I was getting to the point where I just wanted it to be over, which is never a good thing. Then starts the final phase, the battle on foot. This should have been the best part of the game, like in WW. It really should have. But it wasn't, Ganondorf could block almost all of your attacks, his attacks consisted of walking toward you really slowly and swinging his sword at you. That was it. It was incredibly anti-climactic, and it basically boiled down to walking toward him, dodging his attack, hitting him once, and doing it over again. It was a chore, not a final battle. All the battles with that giant green goblin were better, even the fight with Zant was more satasfying than this. The scene afterward saved the game for me, but that fight was still a major disappointment.

And I'm not done yet. Next up, Doctor Breen in Half-Life 2. Personally, the whole last section of the game just didn't do it for me. Having the super gravity gun was cool and all, but the novelty of it wore off after a few minutes. It's boring when you've got a super weapon that kills anything in one hit, there's no challenge involved. The battles in City 17 with the striders were excellent, you had to use all of your wits and skill to survive, unlike here. But getting back to Dr. Breen, talk about anticlimactic. You don't even really fight him, you just mess up his teleporter. After the aforementioned City 17 battles, I was expecting the final battle to be a fight to the death with some previously unknown Combine enemy, something great. I didn't get it at all, I shot energy balls around instead. I didn't even get to kill the guy directly, which was also something I looked forward to. I wanted to put a bullet in his head. The game had none of that. Otherwise, I completely loved this game.

Since I've typed enough as it is, I'd also like to quickly include the following: Tartarus, the final boss in Perfect Dark Zero, Mehrunes Dagon, and Algol. Kefka nearly makes the list for being defeated so easily, but the satasfaction of finally being able to kill him made up for it.

Good:Liquid in MGS4 this battle reeked awesomeness two old codgers shashing eachothers faces all the while hobbling along and complaining about there corns.
Tachyon from R&C:tools of destruction this battle was hard and beautiful to watch because you are on a comet, the other dude uses lots of lasers and missiles and if you have sense you'll be using the alpha charger(is that what its called? the lombax super weapon.

Bad:Richard dolton from army of two he concucted a devious plan to rid of you after killing his crazy partner and fighting through millions of people then a cutscen of you blowing him up and not in an awesome way just poof and you don't even see him die

Ugly: the dlc that tried to fix army of two's bad ending by incorporating a shitty boss fight theres a guy wearing say a tank load of metal weilding a light machine gun but he is still able to run faster than you.

Good: 16th Colossus (Shadow of the Colossus).
The biggest and the deadliest of all the colossi.

Bad: Zues (God of War 2)
More disappointing than bad. The giant statue was a much better boss. Zues was rather average and wasn't particularly interesting.

Good- KH2's final boss was an epic run that I've replayed at least 8 times, watching Sora slice through skyscrapers showed just how far he's gone and how strong he's gotten.

Bad-Ninja gaiden 2 had it's share of frustrating bosses but I have to say I absolutely hated the Amillidilos, it spammed the same attack, they make you fight 2 of them at once, and just when you think you've beaten one they can explode killing you instantly.

Mines the last boss in shadow of the colossus. There was no skill involved when it came to beating him,just sheer determination.

Bad end boss: Solidus! what the hell man, that was so easy! As if MGS 2 didn't already blow
Good End Boss: It's a download game for the 360 called castle crashers and the boss was moderatly difficult without being too hard and you would think you killed it but he would come back and i rather enjoyed it. It made the game worth playing

Good: Call of Duty 4. Less a boss, more an incredible end to a stunning game. And the satisfaction of plugging Zakhaev in the back... Basically, long story short, by the end of it I was crying. There was a mix of sadness, happiness and anger.

Bad: Portal. GLADoS just never got me. Not the person, oh no, she's freaking hilarious but the battle was just boring, frustrating and formulaic. The pods were funny however and the game still rocks.

Ugly: MediEvil. I'm amazed that no-one ever mentioned Zarok. Thing is, for a final boss, he's piss easy. The annoying bit is when he decides to show his weak spot. But he does get comically crushed by a boulder.

Fugly: Guitar Hero III. I had enough. When I traded in Guitar Hero, I was still stuck on the Devil Went Down To Georgia and failed to complete the game on Hard. You may laugh, but if anyone makes a boss as hard as that ever again, I will quit gaming for good. OK maybe not that extreme.

Good End Boss: Liquid Snake On top of metal gear in Metal Gear Solid
i just loved the whole challange of it when you played it ont he harder difficulty

Bad End Boss: SSBB Tabuu
He was such a gypsy!