For the 360 - Assassin's Creed, DMC4, Lego Star Wars/Indiana Jones/Batman (LOTS of achievements), FableII, Oblivion, Ninja Gaiden II

Most have real deep story lines, Fable and Oblivion can be played a helluva lot of different ways to keep you entertained. But if I HAD to choose just one, I would go with Fable II.