Thin thin crosshairs or a fine crosshair with a target dot along with enough magnification so you can see your target. I have a few with 1/8" dots that are smaller than a pdogs eyeball at 300 yards. If your crosshairs are too thick you will get frustrated pretty fast! At times I will put a hole in the target and I like to be able to choose what part of the 22 hole I want to aim for.
As far as brand , thats pretty wide open. I have a lot of Weaver Grand Slams and like the 6-20 power on my varmint rifles. Any more power than that and I cant hold still enough. Leupold , Burris , Nikon , and Vortex are hard to beat too.
I prefer fast focus eyepeices but have a side focus that isnt too bad either. You'll also want an adjustable objective.