Top Teir: Colt, LMT, Noveske, Larue, KAC, LaRue, Daniel Defense, and LWRC. Can't go wrong with any of them. They all have contracts with the US Gov and held to very strict standards.
I still consider these top Teir eventhough they don't have gov. contracts. They are all in strict accordance with the TDP.
Sabre Defence, BCM and POF.
Second Teir: Stag, S&W, RRA, CMMG, Bushmaster/windam weaponary, Armalite, these are all good rifles. But they are made specificilly for the civilian market.
And it isn't very heavily opinion based at all.