Selecting a barrel can be tough. They are the single biggest part that affects the accuracy of your rifle. With a great barrel to start with, you can use cheap or second hand parts for everything else and still have an extremely accurate and reliable rifle. But there are so many choices. Even picking a length can be tough, let alone contour, gas system length, the alloy it is made out of, the process used, twist, type of rifling, fluting etc etc. Some of the choices of these are easy, some are much more complicated.
If you don’t want to deal with all the choices, I have narrowed down the best barrels by category here.
If you go for what I think is the best all purpose barrel regardless of cost, then get a 14.5in JP Rifles stainless barrel and have the compensator/flash hider pinned and welded or get the same barrel in 16in. It will outperform most shooters at most ranges, you can put a heatsink on it for improved barrel life if you run your gun hot, it is made of 416 stainless which holds its accuracy for a decent amount of use and they are cryogenically treated for what JP says gives more consistent accuracy over different temperatures.
Note: By “all purpose”, I mean you might use it for room combat, you want to be able to hit out to 300, you want to be able to get in and out of a vehicle without too much difficulty, you will run a variety of ammo through it and you want it to last a reasonable amount of rounds. If you want the best value for an all purpose barrel, try Rainier select or BCM in the 16in length. For best price and quality for a premium long range barrel - Lilja 20in. They even stock them so with luck there is no wait.
However, if you have the time and interest and you want to know more about selecting a barrel, just pick a category below...