I've only had my PS3 back for 2 weeks (had the game day 1, just couldn't play it ha), so i'm only a lv. 27 right now...
but so far...
Ferrari 458 Italia. 723bhp ~1200kg and 7 gears is perfect for the medium length tracks (tokyo for example). Definitely far from the fastest cars in the game, but adjusting the 7 forward gears will have it keep up with anything on more technical tracks.
Ruf CTR2 Sport. I wish it was premium (or all Ruf's for that matter) and its more for GT2 nostalgia. But a really balanced car that is still pretty quick.
Toyota 88CV. I only like this car because its the best example of how much tuning helps, at least that i've come across so far in the game. It's default setup is worthless; pushes worse than a stock civic. But adjusting things and playing around with it turn it into an absolute beast. You can easily walk away with any of the tougher races that its allowed in.
honorable mentions: ZR1 RM, Elise 111R, '02 RX-7 R Type S. All where awesome to drive and great for the events they were allowed in. Although LTW @ Indy in the ZR1 proved frustrating the first dozen times.
Worst:
Veyron. As others have mentioned. Its not fun to drive, and isn't given the credit it deserves in the first place. Needs to be premium, no idea how that one was missed. In stock form it can barely manage 120mph through turns at Indy...thats saying something...
standard Mercielago. haven't bought one yet, but agree with others...its a complete sled on special Grand Tour event. I know its really heavy, but they are notorious for ridiculous amounts of grip as far as road cars go. It skates more than any other AWD i've come across in the game so far.
can't think of a third yet. I've bought some dumb cars for no real purpose, like an 02 Mazda Protege (I bought an '02 P5 new way back). You can actually make it pretty decent for FWD races.