Andy - You may as well have asked which is the best religion. There is no best exhaust answer. Tell us what you definitely want and what you definitely do not want in terms of sound, drone, cost, etc. and we can help you narrow your choices down. Every exhaust is a compromise somewhere whether it be price, sound, appearance, etc. The first question I would ask is what is your budget?
Current: 2014 F10 M5
Previous: 2015 F80 M3, 2013 E92 M3, 2013 F10 M5, 2009 E90 M3, 1998 E36 M3