With the military being very specific about what type of projectiles they use, I think their choice of buckshot speaks volumes about what is the best for defense.
With that said, the only shotgun I have is a NEF single shot 20 gauge with #3 buckshot. Energy wise its very similar to 12 guage 00 buckshot loads, but with more (smaller) pellets. I have a stock sleeve with 2 more of the same load, and 3 slugs.
The shotgun is not my primary defensive weapon though.
Never mistake Kindness for Weakness