I've always liked #1 buckshot. In a standard 2 3/4" 12 gauge you get 9 pellets of 00Buck which are .33 cal each. In the same load with #1 buckshot you get 16 pellets which are .30 cal each. Seems to me to be the better load for defensive purposes.
I read some tactical defense article awhile back touting that 0 buck was more effective than 00. The premise was that you get more coverage on the target. However if I had to shoot through cover (drywall, car door, etc.) I'd rather have 00 or 000.