In the world of AR optics, there are only 3 brands you should even consider buying.
Trijicon, EOTech, and Aimpoint.
Trijicon makes the best ACOG scopes on the market, but they are quite expensive. The ACOG w/ RMR is the most versatile optic for ARs and allows for close and long range switching instantly. These will likely cost more than your rifle, but they are awesome.
EOTech and Aimpoint are both great brands as well, and it comes down to a matter of preference. Do you want a wide field of view with a larger reticle, or a small view with a very precise reticle. I just so happen to prefer EOTechs because of the view and durability. It's all up to the user. Play with some and see what you like.
Good luck, and let us know what you purchase
ETA: frick, I just read the OP. I see AR-15 and respond to the thread no matter what
Sounds like Aimpoint is what you want.
Make sure you get some Daniel Defense back up iron sights, and that you are damn good with irons before upgrading to an optic.
This post was edited on 11/28 at 4:37 pm