Which American car company offers the best bang for your buck in terms of power? Chevy, dodge or ford?

And specifically which car and model is it? I'm talking about the most power for the least money

Out of the models you have listed the following may interest you:
Ford Fiesta ST
Ford Focus ST
Chevy Sonic RS
Dodge Dart GT

All relatively inexpensive for performance oriented cars. If I were you Id go with the Focus ST its really cool! But of course do research to find the right one for you.

Ford Mustang is about as good as it gets in terms of value and power. Avoid Dodge -- too unreliable. Chevy Camaro, just OK but nothing special. Of course there's the Corvette but not a good buy for the money.

Depends on how much you want to spend. Out of the pony car trio (Mustang, Camaro, Challenger) the Ford Mustang is probably the best at lower levels (V6 and the new ecoboost model) however at a higher range, the Hellcat Challenger from dodge is the car to beat with 707 horsepower.
even higher, the corvette Z06 goes up against the SRT Viper and the corvette wins.
at the moment, only 1 company is working on a supercar (Ford with the mid engine GT) but considering that it will have around 600 hp, chances are the corvette will beat it on a dollar per horsepower figure as the GT is more designed for cornering.

Ford