What is your budget?
What will you be using the gun for? plinking, 3 gun, HD?
For CQB i like the Eotech XPS models. They have a big field of vision, a small red dot for more accurate shooting, a big red dot for close targets, not mount needed and a small profile. Battery life isnt as great, but fine for us civies. My batteries last 6 months to a 1-1/2 years.
For 3 gun i like a 1-3x or 1-4x traditional thin cross hairs. Red dots cover to much of the target at distance. Like the Weaver Classice V 1-3x or Nikon M223.