I think it's a tough call. Things are going to heat up in the next year or two. Rumors of a higher performace Camaro are floating around, the Mustang V6 will be over 300HP and 30MPG and the V8 for the GT will be over 400HP soon. And I'm sure the Shelby will be even higher than it's current 505HP when that all goes down. The Stang is lighter, and with those powerplants it'll probably be even farther ahead in terms of power to weight ratio and handling.
The Camaro is currently awesome bang for the buck in my opinion. It's V6 blows the others out of the water right now. It's V8 performance is that of an SRT with R/T price, and it is the middleweight of the three. It is supposedly going to get a 500HP engine soon to be more inline with the Shelby.
As much as I love the Challenger, I just don't think it competes currently. It's heavier, more money, a boat in terms of handling, and has less power. I really wish it wasn't Charger based, I think that cripples it as a performance car because of the size/weight. Not a fan of the Charger ripoff interior, but the other two dont make me go crazy either.
Don't get me wrong, I love Mopar obviously. I really want Chrysler to step it up so we have a decent contender to these modern day horsepower wars! I'm keeping my fingers crossed for a serious upgrade to the Challenger.