you don't say if you want just the terminal or the full exhaust...in the first case you can choose the one that looks or sounds better for you 'cause you're not going to have improvement changing only the terminal (instead it's probable that you're going to loose some torque @ lower revs).
For the full ex. the best choice is always Akrapovich or Arrow...they make two series of exhaust..if you really wanna feel the difference you should take the more expensive one...I don't know the prices in USA but here in Europe it's about 1900€ for the kit.
I've got the '04 zx6r and use Shark manifolds and Arrow titanium slip-on but I arrived at this solution after many experiments.
Other good brands are Arata and Yoshimura but I've not direct experience with those.