As far as magnified optics go, for the money, a scope like the Nikon or Super Sniper is tough to beat. If you're looking to spend a little more and get a few more features and clearer glass, start looking into Leupold and similar brands, a little more ($1000 and up) Nightforce, S&B, US Optics.
I have a Mueller APV on a 10/22... 115 bucks for the scope, 4.5-14x. Also pretty tough to argue with in terms of cost vs quality. Should be fine on top of an AR I'd imagine.
As for CQB, I prefer aimpoints for the dependability. I love the EOTech reticule, but I've had 4 SEPARATE units go dead due to batteries on me during range sessions/training classes. Maybe I'm just unlucky. But the micro series aimpoints are tough to beat IMO, and have never done me wrong. Cheaper versions would be something like the Primary Arms. Great little red dots, just lack the parallax-free feature, which you may be able to do without.
I have very little experience with the ACOG type optics. The ones I have used, though, are really quite nice. Seem to be well worth the money if that fits your application.