Man, this will spark some debate. Let me ask you some questions:
1 - What is your budget?
2 - What kind of sound do you want?
As a vert owner, who likes the classic muscle car sound, and a desire for a few extra hp, but not wanting to spend a fortune, I went with an SLP Loudmouth Catback.
You will get the most hp gain if you swap out the stock H Pipe for an aftermarket X pipe.
The SLP was $350 or so.
Visit
www.mustangexhaust.com to listen to tons of sound clips.
Keep in mind that exhaust is a very personal thing...people like different things. The SLP, though, is generally considered to be the loudest catback mod on the market. However, if you add an off road X pipe later it will be unbearable. I find that I turn off the radio when driving on a nice day with the top down just to hear the exhaust.