IMO the Dodge Viper GTS-R Team ORECA Race Car is the best. I just bought it today (It really damage my 5,000,000 cred budget) and I beat the Panoz and the 2J on Laguna Seca
untuned whilst I beat them on Indy with only the turbo changed (High RPM)! If you want a street legal one, it has to be the Nissan GTR.
Best car for drag? Veyron. If there is enough road to get it up to a high speed. Otherwise a lighter car might get it.
For the Extreme Series overall? Not really easy to pick. I'd say use the BMW M3 GTR Race Car for the German events, and the Ford GT LM Race Car Spec II for the American. Use the FGT for the rest (Apart from NASCAR. I can't do NASCAR...).
For curvy tracks it'd have to be the M3 GTR Race Car or Viper GTS-R Race Car. They handle well if they are ridgid and tuned well (I can help with that if you want). The FGT works well, if you can handle it. But it is a bit
wide. And
expensive.
Looks - Matter of opinion. The Mclaren MP4-12C looks cool in silver.
I don't really like old muscle cars. Sorry.
That's what I think.