Really have to know what you have now to even get started. If you're one of the guys who thinks Flow 40s are quiet... well, then all we can tell you is to run open pipes/glasspacks and hope you're in a county that isn't too strict about it. If you have 50 series flowmaster, then yeah, I agree they're kinda lame and quiet.
Also, kinda need to know whether you want the 'angry roar' sound (Flow, Spintech), the 'raspy/menacing sound' (Dynomax, Magnaflow kinda), the high tech/exotic sound (Borla, Magnaflow sorta), etc. And whether you desire the utmost in performance from your muffler or whether your primary objective is sound (some people will trade optimal sound for 5 hp)