Any other exhaust will improve sound. I have had TORs with the cat by-pass and they sounded good. Kind of a raspy, 3 cylinder sound, but it was pretty cool.
Then I heard a Jardine system on Britman's bike, and I HAD to have them. At idle and low RPMs it is just what the big motor on the Rocket should sound like. You can feel it in your chest at idle. Higher RPMs is more of a 3 cylinder wail.. I have had them for a few years now and they just make the bike right. The Jardines can sound a little too loud for some. But pull into your local bike night, and you realize that they are not loud.. lol. They can also kind of resonate in a full face helmet that can be annoying, but I am so used to it that it doesn't bother me anymore. I still wouldn't go back..
D&D is the least expensive route to go. I was going to go that route at one point, but never did. I have never heard them, but others say they sound similar to the TORs.
I LOVE the look of the Zard muffler, but it's rediculously expensive.
Someone will probably mention Predators because they have them and are proud of them, however, they were only hand made by a forum member and he doesn't make them anymore.
Carpenter Racing just came out with a complete system. They are pricey as well, but if I didn't already have my Jardines, I would see about going this route..