The R390 road car. Obviously its not a quick as an F1 or even its race car brother, but for "just" 300 and some odd horses, its pretty quick and handles terrific.
Other than that, my favorites are the F094/S, The Imprezza LM, and even the GT40's. They aren't so hot for racing, but for pure visceral thrills, I really enjoy them.
Come to think of it, the D2 Mercedes touring car is kinda fun. Hmmm, maybe I'll have to wheel that one out of the garage one of these days.