I'd recommend the Trijicon ACOG Reflex. It is easily field zeroed by adjusting the dot to the top of the front sight post. If you use backup iron sights (I have a SWAN Sleeve on my flattop), you can flip them up and look right through the reflex without removing it.
Tritium illumination means no batteries. (I have a Trijicon 4x32 ACOG that I bought in '89 and the reticle is still brightly it) A "killflash" lens cover is available for it and it's built like a tank (same aluminum as the receiver on your AR). It's been adopted and type classified for the M4A1 carbines issued to the Special Operations Forces although I have no idea how many have actually been procured. I think if someone had a flattop carbine, a 4x32 ACOG and a Trijicon Reflex, he would have all of the sighting capability he needed for anything the rifle was capable of. Fast accurate shooting at man sized targets out to 300 meters or so with the reflex and precision shooting to 600 meters or so with the 4x32 ACOG. Add backup iron sights (SWAN Sleeve, GG&G or Knights Armament) and you're ready for anything. The downside is total cost for all of those sights would be about $1000.00.
If you can't afford "the system" decide how the carbine fits into your personal battery and buy the appropriate sight.