As just posted in another thread five min ago (IE there are hundreds of threads, just do a search)
Akra, Graves, 2 Bros, Micron, D&D if you dare, Yosh, Termi, Arrow
...the list is huge and only open to debate between yourself and well you. Everyone on here will have an opinion on an exhast note, but alas only your brain will tell you what you like to hear.
I'd suggest seeing if you could find other bikes with different exhast installed and see what they sound like, as I don't think you are going for the performance value of just a slip-on.