I'd definitely say Bioshock. One of the greatest games of all time. Definitely in the Top 5 best games in the history of evar.
I haven't played Bioshock, but my brother played it, and he said that it was like a Beta for a game i like to call Fallout 3.
Openworld sandbox FPS RPG. Four wonderful words that describe the (as voted by multiple game raters) number one game of the year 2008.
With consistent and immense game add-ons frequently provided of the highest quality, I'd say that Fallout 3 is probably the best game of the decade, no, of all time.
Bioshock is like it, only, the RPG mechanic isnt as evolved. (as my brother tells me)
But really, you can't compare Halo 3 to Gears 2, like you cant compare Fable 2 to Star Wars Battlefront. It's absolutely impossible to compare a FPS to a 3PS, simply because of Cover Taking. I havent yet played a game where the concept of taking cover was as evolved as in gears 2. But, i havent played a game with so much creativity involved in it like Halo 3.
(I mean really, Sandbox and foundry were MADE to have the blocks and crates formed together to make a Halo 3 Burger King)
Best FPS?
Halo 3
Best 3PS?
Red Faction: Guerilla or Saints Row 2 or Gears of War 2 (not a fan of 3PS all too much)
Best RPG?
Fallout 3
Best RTS?
Halo Wars
Best game of all time?
PONG