According to this GamesRadar piece, Xbox 360 has the best games in all but one genre compared to Wii and PS3.
Wii, apparently, has the best adventure games, but 360 beats both machines in third-person shooting, FPS, fighting, sports, platforming, racing and RPGs.
The stats are taken from Metacritic, and presented in some cheery graphs, one of which is made from coloured cones. You can't argue with that.
Very interesting, truth be told. Take a look.