Xbox 360 - Best software support hands down, great first party games too. Xbox Live has evolved into such a great tool for social gaming that the other two pale in comparison. Setting up a party to play a game all night is easy and SO convenient (if a server crashes you can all still talk). Software + XBL = best system.
PS3 - The PS3 has been getting more games, but still lacks as diverse of a library as the 360. Still, I think that the PS3 will likely equal the 360 near the end, because Sony made the PS3 with a long lifespan in mind, the 360 was made with a short life in mind (IIRC)
Wii - Simply put, no games. There are very few must have games out for it, and not many coming out in the foreseeable future. RE4, Umbrella Chronicles, Brawl, and Galaxy are pretty much all that interest me, and I've played those out. Weak online support hurts the Wii too, especially compared to the PS3 or 360. The Wii store is great, but the games are pretty expensive (maybe that's just me though)