Some of the ones I worked hard on:
Xmen Ultimate Alliance: Widowmaker (throw 500 people off ledges)
Ninety-Nine Nights: Reach the Highest Level (Level 9 for all chars) and A-Rank for All Chars
Lego Star Wars II: Game 100% Complete
Big Bumpin: What Happened?, Hat Trick, Mine!, Outclassed!, Not Playing Anymore!.. since those are all online achievements for a game that no one plays. Didn't necessarily "work hard" on them, but I'm proud of em!
Call of Duty 2: Complete the game on Veteran. It was my first game that I got 1000 points on, and addicted me from then on.