Here are my thoughts.
Mustang GT: I love the 2007-09 body style, but I'm not quite as fond of the 2010. Still has a nice melding of retro and contempory styling, and its a great handling car. Compared to the Camaro SS and the Challenger SRT8 the GT is a little lacking in horsepower, but it makes up for it with a lighter curb weight and superior handling. I liken the 'stangs power plant to the small blocks of yesteryear...lower displacement, but higher winding. Overall great balance. Buy this car if you are a Mustang guy, you like your muscle cars a little understated, and if you are a tinkerer. There is no doubt this car will have the best aftermarket support of the 3. P.S., I recommend the Grabber Blue/Black paint combo.
Camaro SS: Who can argue with a exterior design based on one of the most recognizable muscle cars of all time? My first car was a '69 Camaro that my dad and I restored, so of course I'm a little partial to the new Camaro. However, I have seen many in person, and it just seems to be lacking something, so much so that I put off ordering one. I love the interior though, and who can argue with 425 hp? Since this is lighter than the Challenger, this pretty much owns most of the performance tests, and wrestles with the Mustang for superiority on the skidpad. Also, its very affordable compared to the SRT8. Buy this car if you want to blow the doors off of everything else, and want a nice mix of retro/modern styling. Just make sure you order a bright color...the great lines of this car get lost in the darker colors.
Challenger SRT8: No doubt this one screams retro, and it probably commands the most attention when all three are together. However, this car is built on the Charger/300C platform, and due to that it is much heavier and sits higher than the competition. Also, much of the interior resembles the Charger, which is a big letdown. This car pretty much requires that you get the SRT8 version, and last I checked it comes in at the most expensive, at a whopping 45K. Compare that to the 35K Camaro that has equal HP and superior handling, and its a hard pill to swallow. Buy this car if you are really going for that retro feel, and have a little extra cash burning a hole in your pocket. Looks great in black btw.
Honestly, I think I would recommend waiting a year or two on the Camaro/Challenger, I have a feeling that there are going to be some nice alterations made for the upcoming model years once they are able to refine the design. If you are looking at a Mustang, I would perhaps try to find a low mileage used Shelby...the price will be similar to a fully apointed SS/SRT8, and you will get the better styling of the 07-09 Stang, superior horsepower compared to the GT, and of course that Shelby nameplate. I'm indifferent on the various tuning companies...if you can find one that fits your style and isnt a fortune, go for it. Personally, I'd rather do the upgrades myself, and use that Shelby as a base car.