I'd have to say the Silent Hill games definitely bring out a lot of the PS2's power.

As for the Gamecube, I know people will disagree with this, but I think F-Zero GX is the best looking game on the system. I think it's possible that MP2 and the upcoming RE4 push the hardware more, though.

The Xbox... well, let's just see how that DOOM 3 port turns out.