On one of my AR-15's, I mounted a Burris 2x7 scope. The front sight post does not interfere with the sight picture. I also mounted a Burris FastFire red dot sight on a top scope ring. You can see the front sight, but it does not interfere with the dot. A nice package that covers close and fast, as well as offering longer range capability.
The Burris optics package noted in an earlier post would also be a good choice. For the most part, I am not a big fan of ACOG knock offs. I don't believe they offer the quality of the real thing, and cheap electronics can have spotty reliability. I like brands like Burris and Vortex as a minimum to buy without spending crazy money and still maintain reliability. Lots of the real cheap stuff usually works, but is often problem prone.
Larry