I'm a fan of the Borla cat-back system I bought 13 years ago and is still performing great. They don't make the whole cat-back system any more, but the key component, the muffler, is still available. See these threads:
depending on mods and how loud you want it; Flowmasters sound pretty nice but not the muffler you want for real performance. couple that flowmaster with 2 inch or 2.25 inch pipping in and 2.25 or 2.5 out and it will sound alright. I personally have 2.25 in pipping going to a glasspack then 2.25 to a flow super 40 and 2.5 out the back. If I could do it again I would get a super turbo muffler and maybe 2 inch pipping in 2.25 out. Then again i have headers and other crap. Borlas sound good. mangaflows flow good but sound weird to me. It all boils down to your wallet.