AR15s have had everything up to a 30 inch barrel as standard manufacture at one point or another. Today, every common length is available up to 24 inches.
The 30in barrels are pretty much overkill with the 5.56mm NATO, so unless you're using the 7.62mm NATO (in which you'd have an AR10) it's just dead weight.
My preference is an AR15 with a 20in Stainless Bull-Barrel, alluminum lower reciver, Magpul PRS fixed-stock, DPS Tactical Grip, Fail-Zero Bolt-Assembly, Timiny Trigger (3-lb), take-your-pick of full rail, and a Leupold MarkIV 3.5-16 scope (usually attached to whatever thermal weapons sight I can get my hands on). Basically, a DPS Mini-SASS with a few mods. Take the uncertainty out of one-shot-one-kill, know what I mean?