Generically, the 'best' holster: 1) covers the trigger guard so that you don't jump that trigger too early in presentation, 2) holds the weapon securely; 3) is comfortable to wear; 4) conforms to the clothing you'll be wearing with it, you dress for the weapon & carry system.
These criteria apply whether the carry system is ankle, IWB, off body (which I'm not in favor of), OWB, shoulder rig, specialty garment or waist pack. A holster which remains open when the weapon is not in it I think is not absolutely necessary though it is helpful, especially in practicing presentations. In a real world engagement you're not going to be recovering to the holster. What you will be doing is covering down on the threat until LE arrives and, when they do, you're going to be obeying their command to put the weapon on the ground.
You should select at least two different types of carry systems to accomodate whatever seasonal or other conditions that might cause you to change clothes.