I dont know about best, but certainly, for me, the easiest car i own to go mentally quick with is my sl65. Just soo rapid!
At the other end, i have a cizeta project that has become a monstrosity! It can easily win EVERY OFFLINE RACE NOW! XD except maybe the fgt one, but im working on it! Lol joke, but it has turned out so much better than i would have ever suspected possible!
However, of all my cars, my nearly stock prowler handles the best! Stage one weigh reduction, stage 2 na engine upgrade. Race exhaust and race suspension installed. It out drags a 458 (wtf?!?!) And out steers everything. It has soo much poise just now, im taking the last essess before the first longish norschleife straigh at about eighty mph. On SPORTS MEDIUMS!!! Its an amazing car, and i love it now sooo much!
Unmodded road cars? For me, it is either an elise, a mclaren f1, ferrari 458 or, shockingly, the stratos. To whoever mentioned it earlier, i know exactly what youre meaning with it; if you can make it work, it WORKS!