this is not as easy as it sounds. Exhausts are not so much louder than each other but all have different notes and pitches.
they come into various catagories such as deep sounding, raspy etc.
The notes also change significantly if the car has induction kit, plenum spacer, high flow cats, replacement Y pipe, decat pipes, headers, aftermarket plenums etc the list goes on.
As an example the K1 is defined as very loud although the decibel readings are surprisingly low.
All we can do is offer a list as a general consensus starting with percieved loudest first.
K1
DC Sports
Scorpion
5Zigen
Nismo
Too many variables for the list to be accurate though.