If I could have any sports car, it would probably be either the new V-10 Audi R8, Porsche GT3 or Lambo LP670 SV.
But if I had to pick american it would be:
Chevrolet Corvette Z06 (I like the ZR1, but the Z06 has been my dream car forever, and its not much slower than the ZR1 on a track)
Ford GT- Best looking and best performing Ford ever in my opinion, 0-60mph in just over 3 seconds!
Dodge: Dodge Challenger SRT-8 (i know the Viper is a much better sports car, and even the other cars in the Challenger's class out-perform it, but in my opinion it has by far the coolest looks of any of the modern-retro muscle cars.