All of these companies make a pretty good rifle. If you want one for typical varmint hunting, any of them will do but may need a little tuning. Most rifles do.
Having carried an M16 around the world a few times, I believe that with proper care most will continue to function fairly well through just about anything you want to put them through. Few people will ever put their own rifle through the kind of abuse that military equipment sees.
For the average "mall ninja" even the crappy Carbon 15s will do.
To get back to your original question: What constitutes "Quality?" Function? Accuracy? Reliability? Ability to continue to operate after extreme abuse? Give some parameters. Some rifles are set up to be Hummers and some to be Ferraris. Both are good in their niche but suck at the other.
I have had good success with Stag, S&W, RRA, Armalite, DPMS, Bushmaster, and even Colt. While teaching how to use these weapons I have seen just about all of them fail at some point, but that is to be expected of any machine.
Can't speak for anything but the Bushmaster M4A3. It has been a fantastic rifle. 1 malfunction during a 3 day Urban Rifle course consisiting of about 1200 rounds. The only other malfunction I can remember was ammo related (remanufactured ammo).
I use this rifle as my patrol rifle on duty.
I have owned RRA, DPMS, Bushmaster, and Superior AR's and shot Colts in the military. The only ones that gave me trouble was a DPMS and the Colt. The Colt was a M4 and this was a new M4 not a old and abused one. Overall the RRA has been the nicest and the Bushmaster the most accurate.