Roush Axle Back set up Or X-pipes
I had Roush axle backs on mine. Very nice sounding system. My set up was stock though, Catted H, Roush axle backs. Deep throaty sound and it wasn't too loud.
And your question will get a lot of answers because sound is VERY subjective.
Pick your poison.
Mustang of the day,
July 12, 2015