I'm personally fond of my 3" Ultra Flos. The but dyno tells me they flow WAY better than the 40s Flowmasters, but they don't have the drone or interior noise.

with the new combo...

http://www.youtube.com/watch?v=IoV0rg3vuww

a couple with the old combo...

http://www.youtube.com/watch?v=8ECqu21g0QE&feature=mfu_in_order&list=UL

http://www.youtube.com/watch?v=3aiKrwGXFOo&feature=related

I'm sure there are better sounding systems out there...but if there is, I haven't found it yet.