Again, this is another topic that you should do a SEARCH for, it's posted all the time...
But I can tell you I've heard zx6r's with YOSH, D&D, HINDLE, AKRAPOVIC, and I've heard other similar bikes with KERKER, MICRON, MUZZY, MIG and TWO BROS. (and maybe some others if I think about it?)
The LOUDEST pipe I've heard is a Hindle... especially for high-rpm WAIL... it's a screamer.
The YOSH is probably the quietest I've heard... it's sorta DEEP and GRUNTY... but hasn't got the high-rpm scream at all... VERY polished and smooth sound...
The D&D and the MICRON are similar to me. Both close to the HINDLE for volume and high-rpm scream. But deeper overall.
The AKRAPOVIC has a nice sound, but not as loud as D&D and MICRON. 2 BROS is snorty too, kinda like AKRA. MUZZY is grunty and pretty loud.
But I've got a buddy with an R6 with a HINDLE... I want to record it and post it in the Spring for you guys. The R6 is a screamer to begin with, likes to rev, and throw a HINDLE on it... YIKES !!! It's borderline RUDE. I love it... heh... even at 6,000rpm it sounds like it's having its neck wrung. WILD.
I just ordered an Akrapovic for the new bike... I'll post my thoughts after break in is done and I install the new pipe.
"Keep yer feet on the pegs and your right hand cranked."