That depends on how accurate you want your rifle to be? And how much you want to spend. If your just going to shoot surplus ball ammo and blaster stuff like me (most of the time) then most of the companies have an upper to fill those needs. I think most places use Wilson barrel blanks for their lower priced stuff. And I'm sure a lot of the have their own specific chamber reamer design (some alot better than others). If you intend on spending time at the bench hand loading then Bushmaster, Armalite, JP custom, Gunsmoke Ent., RockRiver and probably a few others offer a better barrel for Service Match or varmint, target work, these barrels might be of better quality like Douglas, Shilen, Lilja and others. Some broach cut some button rifled and probably lapped and more time spent on reaming the chamber and throat. And they'll $$ cost more also. Then theres a good assortment of gunsmiths that specialize in AR's that will put together what you want and guarantee their work. Their also very helpful if your having problems. Some of their prices are'nt alot more than you would pay Bushmaster or Armalite for one of their Match uppers and chances are a custom job from a good AR gunsmith will outshoot a store bought. My best shooting AR has a Olympic Arms 24"stainless 1-10"ultramatch barrel on it. It's about 6 yrs. old now but has'nt seen that many rounds, maybe 1500 or so. and it'll hold it's own with 52gr. hornady's in hand loads. I've heard people say good and bad things about most of the companies that sell AR's and parts. My AR I do most all of my shooting with is a 20" Bushmaster A2. and thats mostly with surplus like I shoot in my Mini and SAR3. Good luck on your upper and make sure you do plenty of research maybe see what other people are shooting at the range and see for yourself. http://highpowerlinks.com/ http://www.mikesshooters.com/
read mikes random thoughts on accuracy it's good reading!