The barrel on an AR-15 or AR-10 will make the biggest difference in your rifle's ability to be super accurate. A lot of accuracy is determined by who is holding the rifle, but if your AR-15 barrel isn't designed for accuracy, you may be fighting a losing battle. One of the biggest factors for an AR-15 barrel is the twist rate. It is recommended to research the best twist rate for the length of the barrel, caliber, and weight of the projectile you will be using. In most cases, a 1-in-8 twist rate will be the most versatile for .223/5.56 bullets, but if you are building an AR-15 with a special caliber or shorter barrel length, you may want to look at a 1-in-7 twist rate.
AR-15 barrels can be made of various materials. In most cases, you will want to stick with barrels made out of 4150 steel. There are other variants you can get that will allow your AR-15 barrel to last longer. An easy upgrade is to look into barrels that use 4150 CMV steel; some barrels can also be chrome-lined to help your AR-15 barrel last longer.