You are going to get a LOT of opinions. That said I have the Laser DuoTech. Sounds great deep and throaty, until you really get on it ,then it just plain sounds mean Other good choices: Remus, Staintune, Akrapovic...
Laser Hotcam slip-on delivers possibly the lowest and deepest exhaust note I've ever heard on an inline four bike (and I've heard many from many makes). I think part of this is the delicious exhaust already coming the KS engine. Combine with the Laser and it is a great combination. Fosterrad.com is a source for this exhaust.
You should also consider the Staintune, I installed one on my S....love the sound, looks great (polished stainless, no decals or nameplates), and the simplicity of installation (just bolt it on, no spring thingys). I looked at all available exhausts, this is the one that pulled my trigger....but to each their own.
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