You will get hundreds of opinions as we all think our sounds the best. It will depend on your budget. Do you want Axle back, or Cat back, what is your end goal.
Best suggestion is to check out this thread, sit back with a set of headphones and take a listen. Then find someone that you can check out there sound.
http://www.camaro5.com/forums/showthread.php?t=33104
Me personally, I like this one.