The Wii has graphics and CPU similar to the XBOX 1. It's based off the GameCube's GPU which was in turn based on early Radeon (8000 series or so). The 360 is based on the ATi Radeon X1900/1950, though highly optimized and changed from the actual X1950. The PS3 is an nVidia GPU based on a 7900 or something like that.
The PC theoretically has the best graphics, though it isn't as optimized.