What is the best buckshot for home defense?

Run quality 00 buckshot — ideally a reduced-recoil, tight-patterning load like Federal Flite Control. The tight pattern keeps all your pellets together at across-the-room distances, which is both more effective and safer (you are accountable for every pellet). Birdshot is generally too weak to reliably stop a threat, and slugs over-penetrate badly indoors. Pattern whatever you choose at your real hallway distance.