I vote Wii.
It's not really for graphical power, since we know that it can't beat the other systems....but it is the system I have had more fun on so far this generation.
But then to be fair, I only play a 360 and PS3 at a buddies house periodically...and I only own a Wii. So it wins by default mostly.
By next year, it can be any systems game...since by then the PS3 and Wii will both have their heavy hitters out that can rival the 360's library.