A gun's power, recyclability, and accuracy don't mean spit if it's unreliable. And reliability is questionable if the owner/user doesn't properly maintain the weapon.
Here's my criteria, in order:
1. Reliability: Nothing else matters if the weapon fails to go "bang" when you pull the trigger. Get something that is as reliable as possible.
2. Accuracy - both weapon and user: A .50AE weapon is useless if you can't hit anything with it. Likewise a hit from a .22 can kill if delivered by someone with good shot placement. Get something that you can hit with.
3. Power: "Knockdown power" is a myth, but the bigger the caliber you can accurately hit with, the better.
4. Magazine capacity/reload speed: Enemies do not always come solo like they do in kung fu movies. They also do not always come in convenient six-packs, nor are you a movie hero that can put a bad guy down with just one round and never miss. You may also wish to use suppressive fire. Magazine capacity is an additional weapon - if you have it.