Wii with 360 an extremely distant second.
With Nintendo addressing the issues with the previous system (lack of online support), encouraging greater interaction, offering a more traditional controller for those who don't like the remote style, the smaller price tag, and offering an absolutely stunning group of launch release titles, there's just no other choice for me. No high def support? Big deal, I can't afford a HD TV anyway.
If the 360 drops the price by 100 dollars by the time the other two come out, I may consider it, but the good content for it is extremely limited. Right now: Oblivion, and not much else. Halo 3 was their big selling point this E3, but it may be more than a year and a half away, just like GTA4. XBox Live was a point in its favor, but with the wireless internet support for the Wii it's just not as impressive anymore.
I have to buy my own game systems and games, so the PS3 isn't even an option. Not to mention that Sony has no unified online support, and very little in the way of launch titles beyond Metal Gear Solid 4.
Really, the only thing that comes as a mark against the Wii for me at this point is the lack of Final Fantasy, but I have the feeling that the inevitable crash-and-burn of the PS3 will lead Square to bail out of the PS3-only agreement as soon as they possibly can. Hell, the Wii is already getting a FF:CC sequel and a Dragon Quest game.
But GUYS:
Super Smash Brothers
Super Mario
Metroid Prime 3
DUCK HUNT
Fire Emblem
Call of Duty 3
Dragon Quest
Resident Evil
Red Steel (apparently, Prince of Persia with samurai)
I mean, this is just an obscene bounty of launch titles.