low power variable would be my choice (1x-4x) or there abouts.
I have a Pentax 1.75-4x24 ballistic plex and a Leapers 1.5-6x42 duplex tactical turrets,
both of those have enough power for any range shot I am liable to take.
I also have a couple of Weaver fixed powers 4.75x40 and a K6-6x38

Choice is usually based on price and brand loyalty, my personal favorites are Weaver & Pentax (more bang for my buck).
Other popular choices, Burris, Bushnell, Nikon, Leupold, etc...