You want the best???? For looks, get something in all stainless like the SS Drummer or the Latus (Gbalias) . Carbon fiber is also nice, but pricey if you want to get all matching decor on your bike. Stainless (for me) looks the best. As for horsepower? Not sure how much more you can squeeze out of the twin with just a muffler. Mapping and race filter and blah, blah blah......
When it all comes down to it, it's all in your budget.