The sky is the limit for optics, it is mostly dependent on your budget. There are several good options in the 1-4x, 1-6x, 1.5-8x, 2-10x and so on. A typical recommendation for a high value 1-4x is the Vortex Viper PST for around $499. The SWFA 1-6x looks excellent to me for around $1000. USO has some cool options out now too for a little more. For a cheaper alternative, the Leupold Mark AR 1.5-4x is pretty decent as is the VX-R Patrol. Burris Mtac 1-4x, 1.5-6x both seem nice and are well priced. Nice Rifle, btw.