The Serpa Tactical is one of my favorite for carrying if you tote an M4 or Carbine for work. Having a holster that reliable locks into place and is safely and easily unholstered even if you are jumping a fence or hopping over trenches. The Auto lock is engaged with your trigger finger which is safely out of the way from the trigger.
Blackhawk holsters are one of the most popular holster designs on the market. Their Serpa line consists of 3 different holster designs. The Serpa Concealment for concealed carry, The Serpa Tactical, and the Serpa Duty.