Really depends what you're into..
Shooters: Gears of War series, Brink, Call of Duty series, Battlefield series, Dead Space 1 or 2, Rainbow 6 Vegas 1 or 2, Halo series, Fallout series, Red Dead Redemption, Mass Effect series.
Rpg's: Oblivion or Skyrim(Elder Scrolls series), Assassins Creed series, Fable series, Final Fantasy series.
Sports: newest versions of your favorite sport. I prefer FIFA or Madden personally. Good luck choosing!
1.) Call of Duty (All are good in their own way)
2.) Gears of War 3
3.) GTA IV
4.) Red Dead Redemption
5.) Halo 3
6.) Alan Wake
7.) Dead Rising 2
8.) Silent Hill Homecoming
9.) Ghost Recon
10.) Left 4 Dead