Hmm.
Tough question.
If we're talking hardware, then PS3. Hands down the best, most smooth running gaming console I've ever owned.
If we're judging more on the game library... probably 360 (though PS3 has many exclusives that I have and love/want).
They're both great consoles. The 360 generally has the better controller for action/shooters, whereas the PS3's controller kills the 360's when it comes to fighting games and RPGs (I'm just so used to playing JRPGs on the Sony platforms).
So. Yeah. If you want a near-flawless machine go PS3, if you want a machine you'll have to replace every couple of years (

Wii... I don't own one (and so can't speak much of it), and have spent the least amount of time on it. I dreadfully want Galaxy (1&2), the Paper Mario game for Wii, MP3, Brawl and Legend of Zelda Twilight Princess. But again, I don't own a Wii. XD