That definitely is a tough question.......in my opinion, tuners are best because the accelerate(tier 3) and handle well. But in speedtraps you gotta let your wingman do the stuff!!!
Muscles aren't that bad either, when you master their handling. Besides, their acceleration is just amazing!
Exotics are really great, although the tier 1 and 2 are sluggish and expensive. In tier 3 many people think that the murcielago is the best but to really dominate you need a tuned gallardo as it accelerates and handles better.