That is an impossible question to answer. "Best" can refer to almost anything...best at what? Best looking? Completely subjective. Best at providing downforce? Best at smoothing airflow over the back of the car?
If you're looking for subtle looks, check out the Five Axis rear lip spoiler...looks nice, minimal design, easy install, and fairly cheap.
A little more aggressive would be the Abflug Spiral 86 design (not sure if this is out on the market yet)
