Interesting you ask this on a WRX forum but what the hell. There are a few resident muscle heads around here.
How much are you looking to spend? The new Cobras are supercharged and put out 390hp as rated from the factory. Of course, on the dyno they put out close to that at the wheels.
The Camaros are dead, the GTO is somewhat "alive" but nobody has really taken a liking towards them. Good setup though, LS6 with 350hp, 6-speed, etc.
There are also the Mach 1 Stangs that have the base Cobra motor (w/o forged internals) and have 300hp stock.
Trans Ams are dead, but you can go for that GTO again. You could also get a nice used WS6.
Then there are the imports. The ones these days can really compete. The WRX comes with a turbo 227hp stock motor and can get to 300hp very easily. The STi comes with turbo 300hp stock motor and can get to around 350+ very easily. You also have the EVO which comes with about 270hp turbo motor and can make great power. And if you dare, there is the *gulp* SRT-4 which comes with 240hp turbo motor stock and can make great power.