The Shelby GT350R will always get my blood pumping, I love small-block American V8s built to rev. The GT40 and C2 Corvette racecars also fall into that category (the '69 Gulf GT40 used a 302 motor).
I'm really going against the flow, but the '67 Mercury Cougar is also a favorite. The FE big block is known for the 427 and 428, but the 390 was a respectable engine in its own right.
Lancia Stratos, it's got a Ferrari V6 in a rally car. Do I need another reason?
And of course, the quintessential Lotus Elan! If you've driven it, then you understand. If you haven't, no words of mine will ever be able to describe it for you.
You won't see them raced or even talked about very often, but I have a thing for the classic Alfa Romeo street cars in the game. The Speciale has amazing aerodynamics, the Spider is just gorgeous, and the Sprint GTA is just plain awesome, not to mention the shared 1.6L is a work of genius.
Many of the cars mentioned here go fast and are fun, some are even quite stylish. I don't know why, but so many people who play this game seem to give little regard to the Giulia-based cars and really they are missing some of the best cars ever made! The '65 Alfa Giulia Sprint GTA is a real hoot to drive and outside of a 300SL or E-Type you're going to be hard-pressed to find a car in this game that just LOOKS SO GOOD driving on a mountain road!