GTO - PERFECT SLEEPER! LS-series engine is practically bullet-proof, not to mention that, hey, it's a Chevy/GM small-block V8 - simply put, the best V8 for muscle (Hey, there's a reason why even Fords put the GM smallblocks into them!)
SRT-4 - Fun little car - when it actually runs. But it's a brutal ride that is really cheap - in two ways (price and quality.) Plus, it's not even AWD, let alone RWD.
Mustang V8 - Unless you're talking their top Shelby's/GT500's, they're over-rated, under-handling, and while they've got some decent power, they're simply slower than the GTO's. Not to mention far more common.