Here are your options, Call of Duty 4, Bioshock, Halo 3, Assasins creed, Super Mario Galaxy

Halo 3.

it depends do you want the best multiplayer game or best stand-alone
or maybe best online play or best storyline

well by online play its halo 3 hands down
for stand alone there is no other option but assasins creed
when it comes to multi-player its call of duty
and best story line is bioshock
and if you want best family friendly game, super mario galaxy

Bioshock

I wish I could say Gears of war, but that came out in 2006. So I would go with Halo 3 because I have Super Mario Galaxy and I thought it was just an OK game. I also have Call of duty 4, that is getting extremely boring after playing so many hours online and hearing, "Our UAV is online", millions of fricken times. So I say Halo 3. Cause that game is AWESOME!

Halo 3 or Assasins Creed