My friend and I have built over 20 AR15's together. So far our favorite configuartion is a 14.5" carbine, utilizing a mid length gas system, Battle Comp, Daniel Defense 2pc bolt on quad rail, low profile gas block, pop up BUIS, and MagPul furnature w/ an upgraded trigger.
We mostly use BCM parts, and lately have been buying lowers from Palmetto State Armory for 50.00.
While not the most accurate of configurations they tend to be the most reliable. His favorite 14.5 middy has over 1500 rounds through it and we haven't really given it a "proper cleaning" yet.