It really depends on you....so tough to tell from sound clips on youtube IMO. From what I've seen/read/and heard Eisenmann & Remus sound the best with Magnaflow (US) following behind. But another thing I've noticed is a lot of the people in the UK (to include you Irish folk as well as the Scotts) tend to not prefer the louder throatier exhausts.
All in all, having heard a good number of the various exhausts out there I would rate the best sounding as
#1 Eisenmann
#2 Remus
Note: I still haven't heard the Magnaflow in person on a Z4....they shound great on a 2000 Chevy S-10 pickup with 4.3L V-6 w/ vortec engine though. (Its what I put on my S-10 when I was in Vegas)