"Best" is an extremely subjective term, and kind of a foolish question to ask because it's too vague. The answer depends on your end goal. The great thing about the Mustang is the huge support from both aftermarket companies and the community alike, to be able to achieve just about anything you want for personalizing your car.
There are way too many options to cover in a single post, so instead (I'm sure others have touched on this) I'll point out the obvious. X-Pipes typically give a more aggressive, higher tone during higher RPM's, while H-Pipe's give a more throaty, deeper tone during lower/higher RPM's.
I personally run the Roush Axle's with a Lethal Performance H-Pipe and the note is exactly where I want it. It is loud, louder than I actually expected but I enjoy it. There is zero drone under 70 mph and minimal levels of drone for highway cruising.
Your best bet is to not rush into it and view as many videos as you can, go to as many car shows (with heavy mustang populations) to get an in-person listen, as you can. It helps a lot.