There's quite a few that I like:
Tickford/Ford XR8 Race Car: This thing looks amazing in the replays, maybe becasue of the contrast thw white car has against the scenery. Anyway, it looks amazing.
Nissan Skyline R34 GT-R: Either in Yellow or Blue, this car has an amazing 'fierce' look to it, it screams "get the **** out of the way".
Mitsubishi FTO: Cutesy, curvy, and nimble. It's dimunitive size makes it look so out of place against all the Supras and Skylines it can mix it up with.
But overall?
It has to be:
Corvette C5R Wow, thats all there is to say. Those lights, the extra 'nitelites' in the plate recess, the huge spoiler, it all combines to a great package.