Name the game and the achievement, explain why it was tough and why you're proud to have it.
For me, It's gotta be Star 69 from MW2 due to how frustrating the last set was, with the "Speed Reader/That Just Happened" Combo from CEA in second (I got them both in one go... it was pretty tough)

Hmmm.

I have two from Borderlands.
Braaaaaaaaaaaaains!  -Because it was a bit annoying (due to me accidentally setting zombies on fire with my Phoenix skill) and time consuming.
Sneaky Little Buggers- It wasn't really all that difficult, I just figured I would list this one because it did a good job of making me hesitate when opening any dumpster/box/locker/chest/etc.

As far as why I'm proud to have them, eh I guess it's because I'm glad I stuck it out instead of just giving up on both of them due to them taking time.

one 4sakenangel said:

Hmmm.

I have two from Borderlands.
Braaaaaaaaaaaaains!  -Because it was a bit annoying (due to me accidentally setting zombies on fire with my Phoenix skill) and time consuming.
Sneaky Little Buggers- It wasn't really all that difficult, I just figured I would list this one because it did a good job of making me hesitate when opening any dumpster/box/locker/chest/etc.

As far as why I'm proud to have them, eh I guess it's because I'm glad I stuck it out instead of just giving up on both of them due to them taking time.

YES!  These for sure.  And  "The Collector" from the Claptrap DLC as well.  

Currently the bobblehead one and the pizza's from Oh What a Party one as well are shaping up to topping the list...  

Prince of Persia.  What you have to do, is press start. Soooooooooo difficult

Mister B said:

Prince of Persia.  What you have to do, is press start. Soooooooooo difficult

Please, I got Lego Star Wars 2 about 4 years before i even got a 360. Havent played it once but im already proud of all the achievements i will get in the future.

But the hardest achievement I have ever gotten is from Halo: Reach "Skunked". I recently acquired it when i was playing a game with some people i met online and we decided to play some Invasion. 4 people from the other team left making it an easy win. The reason I never had it before was because its just impossible to set up Invasion without wanting to play something better

Most definitely "The House that West Built" from Splatterhouse, it was annoying when the game would glitch and the clock would run in between waves in the arena, adding about a minute to the clock in the end, especially on the fifth arena, making it almost impossible to get