I gotta put my opinion in here. For sheer time spent playing a game, then Oblivion has my vote. I have over 200 hours playing that game (I'm sure that is little compared to some). I can pretty much play that game anytime and have something to do. I havn't even finished the main quest yet!
If you were asking me for the best game for fun factor, then I would say Gears of War. I played through like 5 times (twice on insane). The graphics and gameplay are just amazing.
My latest fav would be Crackdown, I mean being superhuman is just plain awesome. And I pretty much like all games that you can level-up stats.
And finally, MLB:2k7 has the coolest graphics for a sports game. The gameplay is pretty standard, but just watching is fun. It looks like a REAL game in HD.
Lost Planet, Tony Hawk P8, Dead Rising, and COD3 would be my honorable mentions.
BTW, this list will be getting HUGE this year with all the sweet games planning on releasing.
game on.