That's an easy answer. An EoTech of one flavor or another. However, that may not be right for you.

EoTech sights offer no magnification. So, if your eyes (like mine) aren't what they once were, the EoTech may not be the way to go for you. But I don't shoot mine at great distances...except just for fun, so magnification isn't an issue for me.

If money was no object, I think I'd equip one of my ARs with a Leupold Mk4/CQT. It's a 1-3x scope with a nicely illuminated aiming reticle. Perfect for an AR rifle that will be shot to intermediate ranges...say 400-600 yds. But it works equally well at close distances as a CQB type sight as well. The downside is...it's a little heavier and bulkier than some other optics like EoTechs and Aimpoints.

One piece of advice I'd give...don't skimp on quality when it comes to mounts and optics. You probably paid in the $800 - $1000 range for the rifle and ARs are frequently capable of substantial accuracy. Don't put the cheapest thing on it you can find if you're looking to take advantage of the potential the rifle has to offer.