By this I don't just mean the best thing that you've personally accomplished in a game, I also want to know what you think is the greatest thing you could ever do in any game, ever.

So the thing I'm most proud of in a game is getting the Vanilla Crazy Cake from Portal;
but the thing I'd love to get as my "trophy" of gaming that I haven't done is beat Ninja Gaiden II on the hardest difficulty.

What about you?

[EDIT]
Oh, and I don't just include the 360's "achievement system", so apologies for the misleading heading. I just mean something in a game that was hard to do, or something that you think will be hard to do that you haven't done yet.

The thing I am most proud of is beating Ninja Gaiden: Black on the hardest difficulty (Done it yesterday yey me)

The thing I would want to do is get the seriously achievement on Gears, that thing is sooooo long.

i believe my largest accomplishment of all time in gaming is getting the inhuman achievement in Guitar Hero 3 in case you dont know you get this achievement by beating through the fire and flames of expert mode and may i say it was worth it and F#!@ing hard!

little rocket man on HL2:2, god that glitch was a pain in the ass and he doesn't know how to f*cking buckle up so every time I hit teh brakes he smashd his head on the dashboard...then out of the car..then I had to go and retrieve him whilst a chopper was shooting me and Gerald* out of my arms.

*He's my gnome and I can name him whatever the f*ck I please.

My biggest accomplishment came when I beat Street Fighter 2 turbo edition on the fastest speed setting and the hardest difficulty level without dropping a round (a perfect game in other words). Those where the good old days man, cause while I can still hold my own on any Street Fighter game that still was the pinnacle of my awesomness on those games :).

Figuring out how to use the "monkey wrench" in Monkey Island 2... that puzzle is seriously hard to figure out as a 12-13-year-old boy whose 1st language isn't English :P

pop9888:
i believe my largest accomplishment of all time in gaming is getting the inhuman achievement in Guitar Hero 3 in case you dont know you get this achievement by beating through the fire and flames of expert mode and may i say it was worth it and F#!@ing hard!

Yeah, any achievement involving GH3, expert, and lots of time is a pretty big gaming achievement.

For 360 I'd say that finishing GH2 and 3 on expert, and finishing DMC4 on all difficulties are the hardest things I've done, as for most rewarding...I can't really say, I got every single achievement on Bioshock, and I felt that utterly completing that masterpiece was rewarding.

conqueror Kenny:
The thing I am most proud of is beating Ninja Gaiden: Black on the hardest difficulty (Done it yesterday yey me)

That...that's just not possible!

my best achievement is the one where you had to survive for 60 minutes on GRAW2 online map. absolutely manic at the finish.

i wanted and briefly tried for the COD4 mile high club one. but was never gonna get. and now im far to rusty to even attempt it.

my best achievement is that i dont care about achievements, id rather worry about finishing a game.

For pressing start in the Simpsons game. 5G. It felt so good.
I don't know why I bought that game. Probably should return it.

I'm pretty sure a good majority of the Portal achievements are impossible to get. Those would be pretty great.

If it were any other GTA game, I would feel good about getting the 'Survive for 5 minutes with 6 stars' achievement on GTA IV. But since there are no longer tanks coming to kill you, it didn't feel like much of an accomplishment.

I still liked GTA IV, though. I thought it was worth my hard earned monies.

Completing HL1 for the first time recently, considering compared to today the graphics are TERRIBLE! Plus the repeated dying, and having to just persevere through the majority of the time.

pieeater911:
If it were any other GTA game, I would feel good about getting the 'Survive for 5 minutes with 6 stars' achievement on GTA IV. But since there are no longer tanks coming to kill you, it didn't feel like much of an accomplishment.

I still liked GTA IV, though. I thought it was worth my hard earned monies.

Its a pretty rubbish achievement, just piss off the cops and fly off in a Chopper. Or hold up in Tw@t internet shop with its strangely bullet proof glass with a car blocking the door.

Or you could just go into the subway.
I don't know why people insist on calling the place "Tw@t." The @ already has the t, so there's no need for it. And on top of that, the sign above them says "Tw@." With no t. Why does everyone call it tw@t? At first I thought it was "two" but then I realized it was an @ sign. Never once did I think "twot."

Not literally an achievement (ie didn't get gamerpoints) but 5 starring One on Expert on Guitar Hero III.

5 Starring My Name Is Jonas on medium in GH3 on my first time ever playing. I was with two friends and we dedicated the night to it. (I was half way to being drunk when I played the song, I always get better at that game the more i drink). We couldn't remember the lyrics so we made up our own while we were riding on the train to the club. Great night. Most fun I've had with a video game in a long time.

Shadow Tyrant:
Or you could just go into the subway.
I don't know why people insist on calling the place "Tw@t." The @ already has the t, so there's no need for it. And on top of that, the sign above them says "Tw@." With no t. Why does everyone call it tw@t? At first I thought it was "two" but then I realized it was an @ sign. Never once did I think "twot."

Never heard of the word Twat? Thats the joke of the name.

Killing every killable creature(excluding quest characters) in oblivion before any of the creatures respawn.

Playing through Ravenholm on Hard without using my flashlight. scary stuff....

Also tanking through a whole room of enemies in ST when I used to play WoW. The Prophet boss room....nearly pulled him :p.

Finishing the Bank Robbery Mission on GTA IV on my first try.
Or
Finishing Kingdom Hearts II and only dying once.

Getting the feirce diety mask in LoZ:MM.
YOU NEED TO GET 20 MASKS!
WHICH INVOLVES ANJU AND KAFEIS QUEST!

all the puzzle Achievements in PGR 4, in fact most of the PGR 4 stuff, like getting 2 bikes simultanoeously doing endos exactly 250m away form each other....that was more a pain.

Traveling all the roads in TDU is also up there, took me 4 weeks.

Beating the cave of ordeals on Twilight Princess was awesome
Getting Biggoron's sword was cool too
I'd really like to beat Silent Hill, I'm only at the school and I've completley wussed out-_-

Defeating Majora without the Fierce Deitys mask. its possible, but hard.

my biggest achievement would be having like 60 - 0 in cod4 online when everybody else has shittyscores like 3 - 5 or something..

And the most impossible thing i could think of was completing tomb raider anniversary... that game sucks.. i regret buying it -.- it was fun (despite the wicked camera angles) but when i got to the first boss (dinosaur monster or whatever) i used like 20 minutes to get it to 80% health and then i just put that game back in its box...

godinshapeofadouchebag:
...

...wow...what a great name...

Coming in first in 5 BF:Vietnam games in a row (online)

The T rex? It took me about 5 minutes to figure out then 30 minutes to kill properly. You have to get it to headbutt the wall. Makes perfect sense.

Completing Mortal Kombat & Mortal Kombat II on the Mega Drive on the hardest difficulty, without taking a hit.

MK1
http://www.youtube.com/watch?v=6MsNq3A4I4Q
http://www.youtube.com/watch?v=GOhYMTA5ryI

MKII
http://www.youtube.com/watch?v=X8_W9dFK_XE
http://www.youtube.com/watch?v=XC_5ywfaAoU

My most glorious moment in gaming was when [ Ashes of Al'ar ] dropped on my guild's first time downing Kael, and I won the roll. I went absolutely berserk with glee.

Perhaps not a testament to my skill as a gamer, but a great moment nonetheless.

or perhaps

Codgo:

Never heard of the word Twat? Thats the joke of the name.

Yeah, no shit. But the t at the end is unnecessary, as the @ symbol covers both the a and the t. Hence "at." Otherwise it would be pronounced "twat-t," like you have a stutter or something.

I know everyone who owned the game attempted this because it would allow you access all the multi player extras.
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vinus

Wide Circles

Wide Circles

The greatest gaming achievemet i've ever heard of, which im not sure is even possible. Resident Evil 2, Leon Part A with only using the knife. Honarable mention for Ruby Weapon on FF7.

Considering the 360's system some of the crazier things i've done have been: Saint (Dead Rising) - Evacuate 50 survivors out of the mall, Treadstone Assassin (Bourne Conspiracy) - Beat The Game on Assassin difficulty, One Man Army (GTA 4) - Survive for 5 minutes on 6 star wanted level. I probably have some better ones but I seriously can't place them right now.

Other things I know i've done include beating Super Ghosts and Goblins, Kingdom Hearts on expert without dying, Max Payne on DOA. Also beating X Men and Spiderman vs Arcade on the SNES. Max Stats on all characters on FFX as well. Probably lots more but those tend to stand out.