PC is best for RTS, RPG, MMO, Racing games, Flight Simulators and FPS games do to the controller (keyboard/mouse), mod-ability and better graphics.
Xbox and Ps3 are about equal when it comes to fighters and platformers with the main difference being the controller which is a personal preference thing.
The wii kind of does it's own thing.
I have no idea about sports games as I don't play them.
Personally I would rate the PC to be the best gaming platform overall.