LMT. But note that they pretty much only have 16" w/ carbine gas system uppers as far as the normal uppers go. They also have a monolithic rail platform.
The other manufacturers listed all cut corners on their commercial rifles. LMT builds them near milspec, right close to Colt quality (notice that they are missing minor things like parkarizing under the front sight base, taper pins for the front sight base, double shielded hand guards, and an H buffer). But they do stuff like proper gas key staking, proper buffer tubes, magnetic particle testing bolts, etc.
Rock River cuts a lot of corners. I used to like them a lot, now I don't. They cut some serious corners. They don't do any of the bolt testing. That's the little stuff that matters to serious users. Bushmaster is very much the same way. Some manufacturers (like DPMS) seem to not know what a 5.56 chamber is, and their chambers are actually cut for a .223 chamber.
Colts are more expensive. And they are in high demand. I don't agree with Colt's politics, so I don't buy their products. Other manufacturers make 1911s and AR-15s, so I am not dependent on them.
Colt. Nuff' said. In reality though, they're all pretty much the same
Not all ARs are created equal, no matter how much someone will try to say they are. The devil is in the details. Most brands will work fine for a casual plinker, but when you are taking intensive classes or placing your life on the line, you would be smart to buy higher quality rifles and parts. I will trust any of my rifles in a fight, because I don't buy the lowest cost weapon available.
I'd go with the Charles Daly AR or an LMT for that price. Nothing else, except a higher priced Colt, Noveske, or similar higher end AR.
