What is the loudest exhaust for the r6s. I also heard you can use 03 to 05 r6 slipons is this true ??

The loudest exhaust is a matter of opinion. And yes, the S-model "is" the 2nd gen R6 so any 2nd gen slip-on will work on the S.

i had 2bros it was loud and deep. i just got a GYTR full titanium and it is deep, but not as loud as my old 2 bros slip-on