There are so many brands that people SWEAR by. So your going to hear alot of people saying whatever they have is the best. Heres a small list of ones that seem to make people happy.
Yoshimura, Two Brothers, HMF, Vance and Hines, Competition Werks....and on and on.
I have the Yoshimura, I like it just fine. You need to re-jet the carbs when you change out the exhaust so look forward to spending another 80 or so on the dynojet kit. Anyways, go to youtube, listen to a bunch of exhaust clips, and buy what you think sounds best. Thats really all you can do.
First of all I'd like to address what NCSTEVE kind of got into. It completely depends on what you want to hear when you twist your wrist. There is no 'better' exhaust system.
There are 3 main things you should consider (IMO):
1) whether you want a slip-on or full exhaust system. I recommend getting the full because you get that extra hp boost. And trust me, you feel the difference! (you must re-jet)
2) how you want the exhaust to look. ex: carbon fiber etc.
3) and most importantly, how it sounds.
I have an Area-P full exhaust system on mine. To me, it is average. I'm not really impressed with the sound. Mostly because it doesn't scream it has more of a ducati/cruiser rumble to it.