BTB = simply awesome.
Milltek I like as they are subtle and sound good, more so at higher RPM
B.Flame I find quite loud personally, had a car in today (gti) with one and could hear it from miles away when customer went for a post map test drive.
Remember that exhaust volumes and sound change drasticly when the engine is tuned further, so its likely to sou d different as you add more mods.