this is not saying what the hardest achievement youve gotten is, but in general
good candidates:
COD4:
- Mile High Club
CODWAW:
- Go Get Some Sun
- The Sum of all Zeros
Guitar Hero 3:
- Now Thats Impressive
- Big Ol Pile of Wins
Halo 3:
- Overkill
- Two For One
- Steppin Razor
GH Smash Hits:
- Bloody Feet
- Amazing Rockstar
Rock Band:
- Platinum Artist
- ANIMAL
- Virtuoso
Rock Band 2:
- Bladder of Steel
- Platinum Artist
Left 4 Dead:
- Nothing Special
- Untouchables
- What Are You Trying To Prove