Mine is at the moment a white Toyota A86 in specs, similar, to what Keiichi Tsuchiya's works TRD AE86 would be like (about 225hp, under 800kg, level 3 wide body - not the GT works body).
This machine is just pure enjoyment on short, tight courses.
Another favorite is at the moment a similarly tuned level 2 body kit Mercedes-Benz 190E - this machine handles just beautifully!
The 500 hp AE86, you win early in the game is another pure bliss machine - pretty fast and just a great challenge, to drive cleanly.
Once, you go to the edge, where you slightly get over the grip level of the rear in corners, touching the curbs, this is just an insane driving machine!
I am not so hot about the fancy modern cars, especially the super cars, which seem to be there for good measure as an NFS title.
The "classic" cars are just wonderful in S2U (the Mazda RX7 FC is another great car btw).
I am tuning a Cayman at the moment - it has fantastic handling out of the box in S2U, which is not a given, and might turn out as another favorite.
I spent most time, winning events in a tuned E46 M3 - great car with great sounds (those break disk squeaks are beautful!) - it is not the fastest but another one with wonderful, neutral handling.