i like the flowmaster sound on classics... and thats what i'd go by. reason being... even the best performing aftermarket exhaust, on a 500 hp engine with all else being equal, probably only makes 10 more horses at the wheels than the worst performing aftermarket exhaust. there really isn't much difference, so go based on the sound you like.
Flowdaddies are one of the last in my book. They are tin sounding, everyone has them, and seem to be one of the poorest mufflers performance wise. And I think they seem to sound more quiet at WOT and never liked a flowmaster wide open. That's my personal opinion. My vote for the best is a toss up between Spintech pros and Magnaflow packs.
We're talking exhaust system and not just mufflers right? If so, then an oval setup with some Spin Tech mufflers. For a great street setup you want all mandrel bends, an X-pipe, and a good set of mufflers. I like the Flowmasters. I have 2 chamber on now and I am going to 2 chamber delta flow Flowmasters. Spintechs are just too loud in my opinion otherwise I would go with them.