PC, purely for the improved performance, customisation, visuals/physics, mod support, superior online etc. I still play my 360 more. I play my Wii a little, but not even close to PC or 360 times. My PS3 sits there as a glorified DVD player (or at least it did until it broke, useless pieces of shit).
That said, I still plug my 360 controller into the PC for when I'm playing games like Fallout, Mass Effect, Crysis etc. Offline FPS games get the controller treatment too, since there's no requirement for mouse reflexes against AI.
So, which do I prefer playing? PC. Which do I play most? 360, followed fairly closely by the PC.