Lets see..
C6 ZR1: Excellent car, I'm biased though, as its my favourite car both in game and in real life, its a blast to drive though(on sports tires, racing tires take away all the fun of the ZR1).
C6 Z06: Another excellent car, at 85,000(?) its a bargin compared to the ZR1, so with the right tune it should be able to match the ZR1, if not beat it. Another one of my favourites
C5 Z06: Not as good as the the two C6's, but still good, with the right tune, it should hold its own in the 500PP and up races.
The C3 convertible is an honorable mention, its an excellent car to drive and the interior view is awesome, its also pretty useful for classic races and can beat all the other classic muscle cars, and hold its own against the Cobra and AC 427. Don't bypass the C2 Z06 Racecar either, its standard(if that matters to you) but its a blast to drive, I've been looking for one since I got the game and it finally popped up today, its quick as hell and its a challenging, but fun and rewarding car to drive.