If you want it loud and don't mind it being raspy, then a muffler delete such as SLP Loudmouths is the way to go.....no mufflers=LOUD. I have them on my '05 GT with everything else stock and they are pretty good. A little quiet for me at idle, but I plan on doing L/Ts and an offroad X, so once I get that done I will see how it sounds and add mufflers/resonators as nessecary.
The reason for mufflers/resonators is to tune the sound to get rid of the raspyness and give it a deeper tone, and also to cut down on drone for highway driving. If you don't care about those things, and just want it to sound as aggressive as possible, then a muffler delete system is probably what you want....