Call of Duty Modern Warfare and Crysis, both for graphics.
CODMW was impressive graphically because it was so efficient. I couldn't believe how smoothly it played how good it looked given the hardware I had at the time. Now with MW2 the engine really shows its age, but it used to be pretty amazing.
Crysis, well, it obviously didn't play smoothly on my 8800GTS 640, but man, it looked great. The first time I picked up one of the squirming beach critters or dove down in the water to look at the burning flare I had just thrown, really did blow me away.