Who is the best AR 15 manufacturer?

COLT has been making the AR-15 for many years before everyone else started making them, so I would have to go with Colt. Yes, they are really expensive, but they know what they are doing since they are the original makers of the AR-15.

If you don't want to spend that money, I would go with a DPMS or Bushmaster. I have a DPMS and love it, but I still think Colt will out-do any AR-15.

Hate to burst your bubble, but there really aren't any AR-15 manufacturers left in the US. Nobody manufactures every component themselves - not even Colt. Their receivers are outsourced to Continental Machine Tool, who also manufacture the receivers for Wilson Tactical, Noveske, Century, Rock River Arms, Smith & Wesson, High Standard, and several others.

Lewis Machine Tool manufactures receivers which match any receiver made by Continental in quality. They manufacture receivers for LMT, Knight's Armament, Barrett, Armalite, and others.

And, like it or don't, no forged receiver is going to be build with the same quality and durability as the billet lowers made by POF, Daniels Defence, Sun Devil/Hero Guns, etc.