I have several ARs for different purposes.
On my AR10 I have a Leupold Mark AR 3-9x40 with mil dots. (nice scope for the money, may get upgraded later to a 10x, but it's fine for now)
On my 16" 3-gun rig, I run a Primary Arms 1-8x24 w/ ACSS reticle. ($320 on eBay)
A couple of my plinking/HD ARs have Sig Sauer Romeo 5 red dots. These little sights are awesome and can be found for $100-125. Bombproof, and great battery life thanks to automatic shutoff/startup. I have them on my 300 BLK pistol, my 9mm PCC pistol, and a couple 16" lightweight rigs.