It would depend on your intended use. I have a 10X optic on one AR and a red dot on another. If you are looking at a middle of the road/do it all optic I'd look into a good 1X4 or a 2X7 variable powered scope.
Nikon P-300 Blackout Rifle Scope 2-7x 32mm Super-Sub Reticle Matte
Leupold VX Hog Rifle Scope 1-4x 20mm Pig Plex Reticle Matte
Nikon M-223 Rifle Scope 1-4x 20mm BDC 600 Reticle Matte
Redfield Revolution Rifle Scope 2-7x 33mm Accu-Range Reticle Matte