This is my list and my list only. This is not based on anything scientific, but just based on my observations. I know I left out some manufacturers, but these are just off the top of my head
Boutique manufacturers: knights armaments, noveske, JP, Larue, LWRC (piston)
Top tier standard: Colt, BCM, Daniel Defense, spikes tactical
Top bargain: palmetto state, windham weaponry, smith & Wesson m&p, spikes tactical, dpms, bushmaster, rock river
Lower priced: rguns, model 1, delton, dpms
Boutique manufacturers: knights armaments, noveske, JP, Larue, LWRC (piston)
Top tier standard: Colt, BCM, Daniel Defense, spikes tactical
Top bargain: palmetto state, windham weaponry, smith & Wesson m&p, spikes tactical, dpms, bushmaster, rock river
Lower priced: rguns, model 1, delton, dpms