flow master is good. But mostly custom setups are the best way to go. Mina, Remus, and Arden are good mufflers. But the piping is important too. Remember, an exhaust system with parts and labor can cost around 2500 for top of the line. Quality parts and whether you take off the resonators or not depend on what it will sound like. You dont want to have an s-type sounding like a honda civic going down the road.
I have a friend with a 97 m3. Beautiful car but his aftermarket exhaust system sounds like a ricer.

 

I have a 2003 with a v6 and manual transmission. I also checked out the prices for after market exhaust systems and most of them are over $1K. Took a chance and decide to have the resonators removed and replaced with straight pipes. Cost $150 and the exhaust note is slightly louder and with the right sound to my ear. Car feels a little stronger as there is better exhaust flow and lost a little weight as the resonators were quite heavy. Certainly not the performance that a cat back system would offer, but I am pleased with what I have.