Assassin's Creed is awesome, but Elder Scrolls IV Oblivion is the best game everrrrrrr. My fav by far. The only bad thing is ive played it countless HUNDREDS of hours and I'm so sick of it there's no way im gonna play it anymore. I want the next one! When are they going to make it!?!?!?!
Halo 3? pshh.. Halo 1 or 2. Both raised the bar when it came to console shooters. 3 added nothing, IMO.
Secondly, the question is too broad, so I'm going to break it down by genre.
Shooter: Halo 1/2 or Half-Life/CS/derivatives Racing game: Gran Turismo 5 Puzzle/mind-bender: Portal Strategy/RPG: Final Fantasy 7/10 Platform: Super Mario 64 or Legend of Zelda Ocarina of Time. Fighting; Super Street Fighter II Turbo or King of Fighers '00 Arcade: Raiden II
For some reason, I feel that the games I used to play when I was younger, the "classics" if you will, are much better than the offerings of today.
Well I agree with crazjayz the question is far too broad for a single definite answer. But my favorite game is Paper Mario for the N64, best game ever.