NSX Type R and Lotus Evora are just so fast, BMW M3 gtr & csl are crazy cars, no bad things about those, brilliant all rounders.
R35gtr is also crazy fast, Scuderia is so planted its superb, ZZII is also a bit special, as is 2010 Impreza, 670 SV and Amuse R1 / GT1 S2000, but my vote goes to the Audi R8 V8 (not the V10).
That V8 R8 Audi is just a ridiculously good car and it's just sooooo fast and easy to drive.
Then again, how do you define 'best' - fastest, most fun, best value for money, depending on your needs and wants, you can come up with alot of different lists really. Especially as tuning a car can make a huge difference, so I think a thread like this should be split between unmodified and modified cars, but again, that's a matter of opinion.
R390GT1 is also superb, but according to the description NISSAN never actually sold one, so technically it never went on a road, so I guess it's an endless arguement as to whether that's a 'road' car or not as it was only ever used for racing.