This is about as loaded a question as you can get in a car forum lol. The best exhaust is always the one you bought, so you'll hear everyone defending/justifying/praising their choice over choosing any other. Some will even try and appear unbiased by saying things like...others are nice, but....
In the end all that matters is which one do YOU like. On this particular car there isn't hardly any difference performance-wise between the various ones available so it's mostly a matter of sound; which one has the sound you like. And that is a totally subjective decision.
Listen to the various clips posted:
http://www.camaro5.com/forums/showthread.php?t=21289
and make your choice, although realize that the sound you hear from those clips can be affected by the microphone used to record them, and the ability of your speakers to reproduce the sound.
I chose Zoomer's because it has the Borla growl I like, and it sounds more like a precision, high-performance race car. It's a smooth, clean sound; not raspy or ratty sounding.