You asked one of those questions that has a different answer for everyone. A exhaust that is very loud for you could be the sweetest sound possible for another guy. You also have to decide how much you want to spend. A full dual conversion with both cats and mufflers could get quite expensive. Several vendors also sell a cat-back setup that only replaces the mufflers and tail pipes.
Flowmaster is widely used, and they have a variety of models available. The standard two-chamber mufflers are relatively loud, but many folks like them. They may be fine if you plan to retain your stock catalytic converters, since the CATs help keep the sound down. You can also buy quieter Delta-flow mufflers from Flowmaster.
I like the sound of the Dynomax SuperTurbo mufflers on a performance car with an H pipe. Its a moderately loud, deep tone that sounds more like a 60's muscle car. I don't know how they would sound on a modern vehicle. Borla also makes good mufflers that flow well, but they are relatively expensive.