I have a Vortex Viper 5-15x44 with mil/mil reticle/turrets on mine. http://www.midwayusa.com/product/183567 ... icle-matte
Unfortunately its discontinued. I'm not a trained marksman but I've been able to shoot 16oz water bottles at 250 yards just for fun.
A decent 2x7x32 works well for me on an AR. They have plenty of power for sighting in and perform well when turned down in the field. This class is not too large nor bulky either.
I put a Bushnell MP on mine. Wish I hadn't picked the x2 cause the FOV sux. Its a nice need dot @ a reasonable price and has held it's zero. Just don't like FOV.