My personal favorite is the Kitaco stainless/carbon. I modified the baffle to more of the race pipe configuration. With a high-compression 137 Honda motor it's louder than I'd like, but tolerable once you're moving. Has a serious thump at idle.
The best cheap exhaust is a modified CT70 unit. Looks stock but the baffle has been opened-up and the outlet is a larger than stock.
From what I've been told, the best flowing pipe is the titanium Yoshimura tri-oval. It's expensive and the muffler is silly-huge for a CT70.