PC by a lot. A lot of my favourite genres are considerably better on the PC than on the console (RTS, FPS, some RPGs) and they're a lot easier to get with services like Steam. I don't want to have to go down to the store or wait for something I've ordered to arrive. I'm lazy and impatient! It's also easier to switch out of a game to do anything else I need to deal with. If I'm playing a console, I need to switch it off or pause it and then go to my computer. I usually use the same screen for consoles and computer, but it takes THREE button presses to get my monitor to switch between console and PC. THREE BUTTON PRESSES.
360 is definitely the next on my list though. PS3 seems to have a few nice exclusive titles, but I've just always found the 360 to be a more alluring console. I like the controllers better, the interface better, pretty much everything better. Hell, I even like the design aesthetic of the 360 more: it looks like a game console rather than a blade server with a fancy enclosure.