Colt is who the military uses but I don't think you'll notice any difference between that and some of the other big names.... I'd stick with Colt, Bushmaster, Stag, DPMS (Rock River Arms) and Lewis Machine & Tool... Bushmaster and Stag both provide some guns for the military but really you're just paying more money for the name with Colt. You can change what are called "uppers", basically the bolt assembly part of the gun and the barrel to a different upper in a different caliber. If you want to learn more the best place to do research is AR15.com
Stay away from Olympic Arms, they're cheap for a reason... There's a guy in WI right now facing felony charges b/c he let a friend borrow his Olympic Arms AR-15 and it misfired and shot a 2 round burst, now the idiots in the ATF are trying to get him for transferring a "supposed" Class III firearm and owning one without a license even tho it was a gun malfunction... Screw the ATF but stay away from Olympic Arms, remember, you get what you pay for with AR's. Colts cost the most for a reason and the others we suggested are right up there with it b/c they're made better.