The titles on XBLA (okay, so they've had a year) are a damn sight better than PS3's version. Wii has some old stacked up library stuff from the Turbografx and Sega that's pretty good. But the game that should shift most peoples opinions about this element of the console wars is Castlevania, Symphony of the Night. Originally a Sega Saturn game but also available on the PS1, it's probably the best in the series (expect to fork out $60 on ebay to get yourself a copy), the creators are relaunching it (hopefully the Saturn version), but only on the Xbox360.
That just makes me want to get a 360, even though I'm a diehard Nintendo fan.