I would have to say any Sabre Defence Industries AR barrels, if you can find them. Check with Wolverine Supplies. SDI specializes in firearm barrels as they had been producing them (M16A4s and M2HB barrels) for the US Government for some time. Although, I'm not sure of they're current disposition in regards to SDI's CEO being indicted and the company undergoing legal scrutiny.
Regardless, they make quality goods. Would still be worth the effort to look for a Sabre Defence AR barrel.