For the X the Eisenmann (sp?) in my opinion sounds the meanest but it comes at a price and rather big one at that. For the 3s (I used to own an E30 M3) I had a B&B TriFlow and boy was it obnoxious but I loved it. Many people used to tell me to get my car serviced because they thought my muffler fell off...they couldn't believe that I paid to have it sound that way.