Colt 6920 - as said will carry a higher premium (i have seen them sell for $1150 around my way), but they will be made IAW the military's TDP, and they will work 100% out of the box
LMT - i still don't think you can buy a complete LMT rifle, however you can buy the complete lower and a complete upper and have a great rifle. I have an LMT defender lower and it has the nicest factory trigger i've ever used. Their chromelined barrels are surprisingly accurate. (approx $1100 [complete lower, upper, bolt carrier, rear sights] some assembly required)
Stag - i like my Stag kit guns, they are great beater rifles, lots of good features, low price tag. ($700-800)
I don't like the current model bushmasters, i've seen a lot of problems with them, and i think their QC is way off for a $1000 rifle.
Rock river arms bothers me with their silly marketing schemes:
ELITE TACTICAL ENTRY - WTF does that even mean?
Chrome lining is not standard it is an option, i don't think they even offer 1/7 barrel twists (which is nice if you plan on shooting 75/77gr bullets). Aside from that, i've shot RRA and they make a nice rifle.
Since you are looking to spend $1200, you can afford the best factory AR's on the market, stay away from any of the kit guns (M1S, Delton) and the low grade bargain AR's (Bushmaster Carbon 15, Olympic Plinker Plus).