It's probably not the fastest, but I own most of the LMP cars and the Mazda 787b has the best handling by a large margin. The engine sound is sublime as well. Definitely my favorite.
Fastest is hard to judge, it's hard to tell if you're really pushing each car equally. The 3 I see mentioned most as being faster than the others are the Minolta, GT-One, and Sauber C9