I really like my setup - Invidia Q300 quad-pipe, though you'd get the same sound with the Q300 dual pipe. It's burly on startup and quiets down when idles at 1500. Upon blipping the pedal and under acceleration, it makes some great sounds, really unlocks the boxer rumble and isn't too loud overall.
Stainless steel, great construction also make it a good choice though depending on where you live, you might not need it.