100% depends what you are after; loud or quiet, decat or standard.
decat will make any system considerably louder.
Scorpian rs192 is suppose to be one of the quietest on the after market
ktec do various systems with and without centre silence
yozza copy is sold by ktec also
standard brand new on ebay is about £350 and will be quiet
this is mine which is the janspeed one on ktec's website, no centre silencer and dcat, apologies for the talking, it's the only one I have of it at full chat
[video=youtube_share;72hPe8Pw8Gk]http://youtu.be/72hPe8Pw8Gk[/video]
this is quite loud all the time. The cat makes it's much nicer but does reduce the performance in the top end.