Active Autoworkes Gen III, Eisenmann with soundpipes, or Rogue Engineering DMS are the three that I'd choose from.
Currently I'm running the AA Gen III.
AA sounds ferrari-ish.. it's a european humming sound... at low rpm's it's deep and burbles sometimes... and high rpms it screems and is pretty high pitched (the ferarri sounding part) it's sweeeeeeeet..
never heard the Rogue in person.. but from clips it sounds great...
only heard Eisenmann on an e46 M3 in person.. all e46 M3 exhausts sound waaay different than they do on regular 3'ers.. so can't give a fair comparo.. HOWEVER, both the AA and Eisenmann retain a European sounding exhaust..
Ohh.. also forgot..
B&B Triflo.. this is definitely the meanest sounding exhaust in my opinion.. it's deep and loud as hell.. it's as close to a V8 as our straight 6's will sound.. they have numerous sound clips on their website... order from bavauto though.. its's like $200 cheaper with them.. dunno why..