Everyone is different, but I'll likely never build a large frame AR again.
6 ARC, 24-26" Bartlein Compass Lake Engineering barrel, Adjustable rifle+1 gas system. SixFive Outfitters or JP bolt. Duramag, E-lander or ASC magazines. Get the stiffest upper you can (Seekins, Aero...) and glue/bed the barrel to it.
That's for PRS or as long range as you can make it... If you want something a little more practical swap the barrel for a Proof Research 16-18". Ammo is out there, LeverEvolution or CFE223 for reloading powder in the 90-110gr range. Whatever dies make you happy. I've been using Hornady standard dies since before the cartridge was released, 10,000+ rounds at this point.