I have used a variety of scopes on flat-tops including Simmons 3 - 9 and 4 - 12, Leupold 1.75 - 6 and 1 - 4. Cheap Weaver Rings were used on the Simmons and the Armalite one piece mount for the Leupolds. I just got a Bushnell 3200 Elite fixed 10 x 40 with mil-dots and target knobs for use on another rifle and I am very impressed considering that it cost about $150 from Natchez. That scope with the Armalite mount would be a cool set-up.
You never mentioned your price range. If you can spend $500+, just think "Leupold" and you should be all right.
If your budget is a little more limited, you may have a couple of different alternatives. You'll want to stick to fixed-power scopes at the lower end to maintain quality and help maintain zero (cheap variables don't tend to do well in either department).
I've never had the chance to use one, but the Bushnell 3200 10x40 that BillL223 mentioned really looks like a nice unit for the money.
In the $300 range, see if you can find a Tasco SS10x42. This was a limited-production military scope that you might be able to find at a gun show or at SWFA.com. I've got one on my Savage 10FP, and I don't think I give up any image quality to the Leupolds I've seen.
Also, count on getting a riser. The AR15 stock was designed for very high sights, and even high rings on a flattop can make it tough to get your eye behind the scope.