The best defensive round is the round you can put on target. I have watched people use everthing from 38 Special to 10mm using the FBI load as a defensive round. Too many people are happy to just hit the black, forget about the X ring, just hit something.
Then there is the group that can nail a target all day long,,,,,,,,, but their gun stays at home, or maybe in the glove box, and is never carried. It's to big/heavy/not allowed/etc. etc.
My word of wisdom is the best defensive round is the round that comes out of the gun that you actually carry, that actually hits where you aim (assuming you aim). A Remington 870, a 45 ACP, 9mm, 10mm, 40 S&W, 41 Action Express, M4, a LAW, or a 500 S&W are all great defensive weapons. A Beretta Tomcat, however, is the best if that is what you are going to carry and you can hit the X ring with it. A "BB" on target is better that an artillery shell that misses.