Veyron is COMPLETELY OVERRATED. It handles like a tank. It's useless. I remember getting excited about this, knowing it's the fastest street legal car in the world, then I took it for a spin and slammed into every wall because the damn thing can't turn.
FGT is overrated. It's like a Superman car: good at everything, basically invincible, but BORING.
Citroen C4 is UNDERRATED. Not useful for everything, sure, but I fully tuned it, gave it a rear spoiler, some hot red wheels, and it looks damn good. It's great for Beginner and Amateur races. Also good if you're into French cars (ME).
Ferrari 599, judging by what I've read so far on GTP, is the most underrated car in GT5. It looks so much better than the Enzo, which looks like crap, and when fully tuned the 599 can beat an Enzo.