Go to CMMG, they are the Burger King of Ar15s and you can have them build it how you want.
Not to mention, they are probably the closest to Mil-spec right behind Colt...which sets the standard.
I have 7 AR15s and they all come from CMMG
As far as Colt and LMT, they are very nice, but overpriced for what you get over the mid range rifles in my opinion.
By the way, since we are on the subject-
The AR is a very modifiable platform, but this is not one of the routes you should go.
The gooseneck thing that sticks up from the bolt carrier is the gas key. It mates with the gas tube to route the gasses into the carrier to cycle the action. The key is held in place by two hex head screws. Those screws could easily work loose with all the pressure, so a good manufacturer stakes them in place (by scoring the heads on two sides) which knurls the key and head and prevents turnout. If the key does come loose, it could cause short stoking (where the carrier does not move far enough to the rear to chamber the next round), because some of the gasses would escape before doing their job. Chemical bonds cannot be relied upon because this is an area of such high heat.
Improper manufacturing would be to not stake them, or to not stake them in a way that knurls the metal and prevents movement.