The best street car is the Tommykaira ZZII, though some have said that it is not eligible for the Polyphony Digital Cup.
The best production-based race car is probably the Raybrig or Arta NSX. I actually like the Viper Oreca and the Gillet Vertigo race car a little better, though.
The best closed-wheel, purpose-built racecar is probably the 787B.
The best open-wheel racecar (F1s) is the F094/S - Polyphony 001.
Look in the "Look Here Before You Ask" thread at the top of the GT3 board to find out where these can be won.
Welcome to GTPlanet, Jubjub! Are you a Simpsons fan?