I don't have a favorite game, however i do have a few favorite game series.
Assassin's Creed Series (They are all good, my favorite would be Revelations), Grand Theft Auto Series (My favorite was San Andreas, however GTA 5 looks promising), Kingdom Hearts Series (KH2 was the best. I'm hoping they decide to make a KH3), Halo Series (This is a tough one because all of the Halo games are good in my opinion, except Halo Wars. It sucked.), CoD series (My two favorites would be Black Ops and MW3. Black Ops because it has Zombies and MW3 because the Campaign was fairly okay.), and last but not least Batman: Arkham City.
Avy & Sig credits go to Hurricane0