There's a lot of schools of thought on this.
Slugs are without a doubt the heaviest hitters, but you might have over penetration concerns.
#3 or #2 buck are the most common defense shells you'll find. But depending on your home situation, you might have overpenetration concerns with these as well.
Some people say you shouldn't use bird shot, but I tried an experiment at the range, and within room size distances... i.e. less than 20 - 25 feet, bird shot (#4) does a great job against soft targets. The pattern is so dense, its really devastating.
My recommendation is that if you live in the country and have no concerns about hitting your neighbors house, and your house is such that don't think you'll be sending rounds into your kid's bed... then slugs and buckshot combinations are the best. Whatever you feel most comfortable with.
If there are any concerns about over penetration, then load it up with some bird shot and learn to aim for the face and neck. You can put some buck shot in shell holder if it makes you feel better.
This post was edited on 9/30 at 8:57 pm