Mass Effect
You know a game is game of the year material when it is so good you keep playing despite the numerous niggles. Bioware once again prove that they are the daddies of the RPG domain.
Orange Box
Purely because of Portal. Genius (note I've only played it on PC but they're the same).
Assassin's Creed
The engine looks fantastic, allows you to go anywhere. The fight scenes and music are top notch. An exhilarating game.
Biggest disappointment - Assassin's Creed
Oooh controversial. Somewhere along the line they forgot to put in a game to match the power of the engine.