Beating Ghosts 'n Goblins (NES) twice in one session. The game has haunted me for years, and to conquer a game that I had told myself repeatedly as impossible was such a major accomplishment.
Other than that, I guess I could say 100% on all trophies in Killzone 2 (Including DLC). The multiplayer gave me some frustration as well as some tedious objectives, but to beat Killzone 2 on Elite was satisfying after that final push in the last chapter (Reminds me of Heart of the Reich from WaW).