I may get blasted for this but spending big a large amount of money for a sight for an AR 15 is a waste. I own one and it has a very basic scope. I make very decent hits with it.
Optics are starting to get really advanced. I think they have a scope now which automatically finds the range then adjusts the reticule.
I still think good quality iron sights are hard to beat. The best I have ever had was mounted on an old Krag. It was a Redfield receiver with a fairly large aperture. The front was a sourdough. That flat gold front really stood out plus the whole thing was pretty durable.
The most successful rifleman I have ever read about was a guy from Finland. I was surprised to note he used only iron sights even for long range sniping.
Of course if he had some of the modern optics available today, maybe he would have used them instead.