Another vote for Leupold. I had an old Leupold M4 that I hunted with when I was young. Once I took a head over heels tumble down a steep boulder strewn creekbed. I remember my rifle bouncing off the back of my head and then bouncing off the boulders down the creek three or four times. One of the bounces pt a HUGE ding in the rim of the eyepiece. I took it to the range and it held it's zero, exactly, didn't stray a bit. With that said, I'd go for a nice tactical scope by U.S. Optics, or Unertl. Something that looks like it belongs on an AR15