Hardest Achievement ever?

So, what exactly is the hardest achievement/trophy you have ever unlocked/seen someone else unlock? For me it was probably The One Free Bullet achievement for HL2: Episode One (can only use one actual bullet the entire game). Also,  what is the rarest achievement you have ever seen?

Global Leader - GRAW
Get to #1 on Worldwide Leaderboards, for only 40G.

Seriously , Gears of War 1

" Global Leader - GRAW
Get to #1 on Worldwide Leaderboards, for only 40G. "

Mr. Perfect (30g) from Mega Man 10. Beat the entire game without getting hit once. Even if you can reload saves to keep attempting stages over and over... Holy shit man. It's just insane.

Get a 5-hit combo in Avatar The Last Airbender. 

I played WoW for a bit, and that Insane in the Membrane achievement looks pretty crazy.

" I played WoW for a bit, and that Insane in the Membrane achievement looks pretty crazy. "

" @DukeT said:

" I played WoW for a bit, and that Insane in the Membrane achievement looks pretty crazy. "

Trust me, duder, you don't even know.  I researched how to do it and looked up websites on how to achieve it.  I remember trying to explain to friends how I was attempting to do it, because I got lost in those types of efforts sometimes.  It's something that no person could do without tons of time devoted to the game.  I owned my own business with lots of downtime and couldn't do it, partially because I didn't have the time of day between other things.  It's retarded-hard.  It's possible and easy enough with a high-level character skill-wise, but the grinding is immense. "

" @Bloviator said:

" @DukeT said:

" I played WoW for a bit, and that Insane in the Membrane achievement looks pretty crazy. "

Trust me, duder, you don't even know.  I researched how to do it and looked up websites on how to achieve it.  I remember trying to explain to friends how I was attempting to do it, because I got lost in those types of efforts sometimes.  It's something that no person could do without tons of time devoted to the game.  I owned my own business with lots of downtime and couldn't do it, partially because I didn't have the time of day between other things.  It's retarded-hard.  It's possible and easy enough with a high-level character skill-wise, but the grinding is immense. "

isnt that the one where you have to do an instance without a single person getting killed? "

" @Ryax said:

" @Bloviator said:

" @DukeT said:

" I played WoW for a bit, and that Insane in the Membrane achievement looks pretty crazy. "

Trust me, duder, you don't even know.  I researched how to do it and looked up websites on how to achieve it.  I remember trying to explain to friends how I was attempting to do it, because I got lost in those types of efforts sometimes.  It's something that no person could do without tons of time devoted to the game.  I owned my own business with lots of downtime and couldn't do it, partially because I didn't have the time of day between other things.  It's retarded-hard.  It's possible and easy enough with a high-level character skill-wise, but the grinding is immense. "

isnt that the one where you have to do an instance without a single person getting killed? "

No.  It's way harder.  Doing an instance at high level is easy. "

dude that Final Fantasy MMO on the Xbox 360 has some NASTY achivements 

Get a Life in UT3. Get 200 kills on 50 different days. Can't beat GRAW but it's the hardest I've seen.

the plat in lost planet 2

what do you have to do? "

Portal's Apeture Science or Call of Duty 4's Mile High Club.