X-Pipe all the way, Dr. Gas is supposed to be the absolute best. It will sound very very close to an H-Pipe until around 2300 RPM, then it gets very smooth until around 4000 RPM and after that is what everyone talks about, it turns into a really loud high-pitched howl that is supposed to sound absolutely crazy and nascar like. So you get the deep muscle car tone down low and the smooth sound at highway speeds (less drone than an H-Pipe) plus that cool sound over 4k.
Mustangtuning and UPR make very good ones for much less too if your on a tight budget.