Well, depends on your preference/price range etc.
Personally, I like low power optics on AR's. For me, my AR is first and foremost a defensive carbine. Not a precision rifle. (I have more appropriate tools for precision). IMO, an AR optic must be usable easily from point blank range, to whatever max distance you want.
I run this: Vortex Viper PST 1-4 X 24mm in a Larue Tactical LT-104 Mount GREAT SYSTEM! I really do love it.
I have shot this system all the way out to 600 yards.
This may be outside your price range, but the Nikon series already mentioned are awesome scopes from what I have heard.
Also, you should look at a mount from Larue, or similar. (Larue mounts are not cheap, but worth every penny) Nikon makes a Larue mount "copy", so does Burris and Weaver. Can't imagine they don't work.
I would say the Nikon scope of your choice, and the offset mount, unless your budget is larger.