Depends on the genre you like. MHO:

RPG - Oblivion, very good Graphics, played thrice through it and even my wife plays from time to time ;o)

Fighting - Soul Calibur IV, gorgeous player models, at the moment no alternative

Racing (Action) - Burnout Paradise, keeps you going for a long time with huge game area, trophy system and online play. Addition of bikes and day/night cycle in the most recent updates makes ot even more worth your whil. Fast and furious alternative: Race Driver Grid

Racing (Sim) - Colin McRae Dirt, good to very good graphics, streetwise nothing in sight until GranTour finallly will appear, but Gran Tourismo Prologue might pass the time

Sports - From what I've read NHL 09, played through the Demo and ordered the title a few days ago, but do not yet have the full version. Also fine: Virtua Tennis 3, very fluent gamplay and very good graphics, though nothing totally astounding.

Shooter - Nothing truly awesome, but Darkness might be a good choice.

Adventure - MGS 5, nothing like it on the market. Bit on the short side, but worth playing twice or three times through. Cut scenes impressive, in-game graphics still very good, story and gameplay awesome.

Cheers!