um, little rocket man: half life 2 episode 2

Mile High Club because most people are like "OMG it's too hard" when it really isn't if you know what you're doing.

Ultimate Player in Pro Evo Soccer 2009, simply for the sheer number of hours I was able to enjoy that game for.

the one in COD4 when you have to destroy all the god damn TVs.

I have one of the "Ally" achievements on Mass Effect 1. 
And I'm damn proud. Oh, and I'll throw the Medal of Valor achievement in there too. Same game.

Don't know it by name, but there was this one skull in Halo 3 of which I had to learn how to grenade jump to get it. I died so many times trying to learn the jump and from the enemies killing me, but it felt so good after I got it and the achievement.

Call of Duty 2 on Veteran, Call of Duty 4: Modern Warfare on Veteran (and Mile High Club), 1000/1000 in Dead Rising, and Bioshock 1100/1100. The full 200 in the original Geometry Wars, too, because it took me over two years to get it.

Mine would have to be the Ashes of A'Lar Mount achievement in WoW, but since I hate that game and everything it stands for now, I would have to pick something else.  
 Wow, I'm the only one on the site who has that one... Crazy. I should be more proud of it (even if it is a good bit luck)
 
100%ing Saints Row 2 is up there, as well as doing it all Co-op. Took a long time.

Off the top of my head, 100% on "I Saw Her Standing There" Expert Bass in The Beatles: Rock Band ... because I did it on my very first try :)

Beating Endless Setlist 2 on Expert in Rock Band 2 is probably the one I am most proud of.  I also have 1200/1200(I think) the first Mass Effect and am working on some other ones(Pure and The Saboteur at the moment) as well to try and 100% them.

I guess Test of Faith in Mirror's Edge.... I really wanted to shoot some people at times.

11,200 kills in GoW to get Seriously 
Mile High Club in COD 4

5 starring all songs on expert in GH3 then in GH2(It is easy now but it was hard back then) 

Saint on Dead Rising ... Such a bitch, damn A.I.

" Aperture Science from Portal -- real tough, great challenge
Head Chomper from Alien Hominid -- huge pain in the ass!
Brass Balls from Bioshock -- not too hard, but it was the last one I needed for completion, so I'm proud of it. Great game, too. "

100 Hours of played time in DOA4.

Finishing the Monster Book and Collector's book in Tales of Vesperia (3 achievements away from Vesperia Master). 
 
Golding every event in Forza 2 is up there as well.

" Off the top of my head, 100% on "I Saw Her Standing There" Expert Bass in The Beatles: Rock Band ... because I did it on my very first try :) "

Mile High Club, Zombie Genocide in Dead Rising, and that damn gnome achievement in Half Life 2.
They stand out for me.

Skill mastering all of the characters in Lost Odyssey. That took a long while.

  "Little Rocket Man" from The Orange Box, "Test Of Faith" from Mirror's Edge &, more recently, "Insanity" from Mass Effect 2. Those are the ones I'll be telling the grandkids about.    

little rocket man and neighborhood watch, and the one bullet from the Orange box, the Templar one from Assassins Creed, Zombie Genocider from Dead rising

Either Brass Balls from the original Bioshock, or getting 100% on Batman: AA. Some of those Extreme Combat Challenges still give me nightmares. 

I'm not sure if I have posted in this thread, but now it's Jiggillionaire!

"Rainbow Six Legend" (beat Rainbow Six Vegas on Realistic difficulty) for me,That was such a pain in the ass to get and it seemed to take forever.

getting 3 stars in 15 spec ops missions solo except for 1.

I got through the 5 and 7 Day achievements in Dead Rising so I'm pretty proud of that. Working on trying to get Transmissionary and Saint in the same playthrough now. Not easy...

My biggest achievements are definitely completing the endless setlists in Rock Band 1 and 2 on Expert. I would just invite a couple of buddies over and plow through all the songs. It was an epic undertaking, but extremely satisfying.

" @MrJared said:

" Off the top of my head, 100% on "I Saw Her Standing There" Expert Bass in The Beatles: Rock Band ... because I did it on my very first try :) "

I need to get that one still, but I also need to get a guitar that isn't broke as fuck too!  lol  I've gotten 97% on it so far.  "Here Comes The Sun" hammer-ons achievement is pretty fucked, too. 
 
Anyways, back on topic... 
 
I'm sure I've commented on this post before, but I'm updating this.  I think my proudest achievement is "I Went Commando" from Bionic Commando, solely because only 5.6% of the people synced up have obtained it, and I also happen to be the fifth quickest synced that got the achievement.  It's nice having a place on that table...and especially since it was my first playthrough on the game. "

Beating every MW2 level on Veteran. It was difficult but a hell of a lot of fun.

Either 5*ing all hard songs or beating expert on GH3.  Beating Gears on insane (solo except for Raam)
 
But being the raid leader and getting the Amani War Bear in WoW is easily the proudest achievement I have. Our guild wasn't that progressed gear-wise and we optimized the shit out of that run to make that happen.  Awesome culmination of a great team effort.

I would say for me it would be the "Better than the developers" achievement in Trine

key to the city in GTA 4, mile high club COD4. 
 
still can't get Wax-off in geometry wars x_x