Gutting (removing baffles) doesn't sound good at all. It's louder but sounds rediculous and is usually obvious. It also has an adverse effect on hp.
I've said it before and I'll say it again, Muzzy has the best relationship with Kawasaki sport bikes. More so in their line of full exhaust systems.
Since slip ons don't really have an impact on performance any of the major brands will sound good. it would then be a matter of preference.
D&D, Yoshi, Brock, Vance & Hines, Akapovic, Two Bros, and Jardin all have quality sound in their products.