I've got to go with the Nintendo Wii. Nintendo's first party games, some good ones from Capcom and Sega, and the occasional hidden gem are nice enough, but because it's backed up by the Virtual Console's ports of classic titles, it gets my vote.
Xbox 360 is my runner up. While it doesn't have nearly as many great exclusives as the winner, I don't have a PS3 nor a PC that can compete with the 360. So, for me, it's the only way to play Mass Effect, Oblivion, and other great games available on multiple platforms. Also, like the Wii's virtual console, the xbox live arcade has a nice selection of classic games, as well as some remakes and great new titles. Also, Halo's a lot of fun.