BioShock for me, with Super Mario Galaxy a very close second.
To me, BioShock is emblematic of the genre mishmash that defined the seventh gen. Part shooter, part RPG, part adventure game, BioShock encompassed (and transcended) several genres at once, and did so masterfully. The shooting mechanics are stellar, the art direction is spectacular, the firefights thrilling, the freedom of choice liberating. It's one of those rare games that keeps you coming back for more, and stays with you long after the credits end.










