Green Mountain was the bare basic
rifle barrel back in the day, and they came out with innovated drop-in regarding TC sidelock rifles. THEN they got government contract work and dried up as far as ML barrels went. Now they are back to making good ML barrels.
Getz, Rice, Colerain continued and continue to make ML barrels. Colerain has a nice variety of barrels including
swamped barrels, as well as smoothbore barrels. Rice also makes swamped barrels. Rice and Getz are considered the top of the pile when it comes to barrel performance, but it appears that Getz is no longer making barrels...at least as of July 2018 ?? Rice for example polishes the bores of their barrels at the factory.
Generally, GM, Colerain, Rice, and Getz will out perform the needs of most rifle shooters, who like to punch some paper and to hunt a bit.
As far as "best" is concerned, that can be subjective. Sometimes what the rifle shooter wants is only made by one of the barrel makers, so they are
"the best". IF the rifle shooter wants a specific type of rifle, a very accurate copy of an original that needs to be built, in the shortest possible time, then the availability of the caliber and style of barrel available, determines the "best", if price isn't a problem. If price is a problem, then the least expensive, but available barrel to meed the shooter's needs is the best.... IF the rifle shooter wants very little if any
"break-in" time for the barrel and is interested in target match scoring, Rice is probably the best. Actually, you really don't get the full application (imho) of what the rifle barrel will do until you are in an artificial environment on the bench at the range.
In real applications, I think (especially with flintlock) the lock and the trigger are amazingly important, before the barrel comes into play. Think about a low quality flint lock or a poorly installed barrel or poorly installed nipple placement...causing less than dependable, and prompt ignition...the barrel means very little in such a case. So just about any of the well established barrel makers out there will give you a very good barrel. The choice of the lock and the assembled rifle will do more for the satisfaction of the shooter, I think, than the barrel alone.
LD