I have a stag arms upper/RRA lower (can see pic in introduce yourself threads look for my name), and I have nothing bad to say about any of thsoe companies. But, if you want absolute best quality Colt is your number one way to go, a colt 6920. Otherwise I would go with an LMT defender lower with SOPMOD and a 16" LMT upper with BCG/charging handle. Buy the upper and lower separate, its cheaper, much cheaper. Inless your lower was approved via form1 get LMTs 10.5 upper, what i plan to do on my next build. You could always go the Noveske route, though they are more expensive then colt, or Sabre, or even CMMG. Top picks would be colt and LMT