anything from 3x magnification and up, preferably variable magnification.
Scope lens sizing preferably 40mm and up.
Avoid the "compact" scopes, as they tend to have crappy eye relief (Distance from eye to scope, where sighting picture is optimal)
I'm running a UTG/Leapers 4-16x44AO SWAT AccuShot Rifle Scope, EZ-TAP, Illuminated Mil-Dot Reticle, 1/4 MOA, 30mm Tube. Easy reset windage/elevation turrets, without need of tools.
(Tube diameter only allows for greater windage/elevation. Havent noticed improved sighting picture)