You'll find opinions here ranging from birdshot to 000 buckshot to slugs. The debate along that spectrum can get rather heated.
My .02 euros.......
Large buckshot 1, 00, 000 gives the best penetration and wounding potential. #4 distributes a large lead payload over a potentially larger area of the torso, but the individual pellets have questionable ability to reach vital vascular and CNS structures. Bird shot, T, BBB, BB, 2, 3, ........6, 7.5, 8 offers questionable penetration capabilities particularly once you get into the smaller shot sizes.
Within distances measured in feet, most loads from a 12 gauge stand a good chance of being effective enough. Once you move to distances measured in yards, then you're larger shot really has an edge.
If you are an apartment dweller who has no control over whats on the other side of the walls in your home, you may have a need to drop to smaller shot sizes to limit penetration and mitigate risk to those on the other side of the wall. You will not eliminate the risk by doing so, but the risk of fatal collateral damage is less. If you are not an apartment dweller and have control over what's on the other side of the walls in your home, then use something effective and control what's downrange in your field of fire. "Know your target and what lies beyond it" and use the most effective load from your weapon to engage the target.
Next comes finding out what that load is......
00 buckshot is 00 buckshot, to a certain degree. All 00 buckshot uses .33 caliber lead balls, some even copper plate it. Some companies use harder alloys of lead than others resulting in less shot deformation as it travels down the barrel. Some companies use advanced wad technologies to help with patterning. Some companies do all of these things. So when you get right down to it, not all 00 buckshot is created equally.
Premium loadings from Hornady and Federal tend to pattern very well compared to other loadings. TAP, and LE Flite Control tend to be top of the heap in performance. Moving down from there, other companies offer reduced recoil and standard velocity loads that pattern very well. Some at the lower end of the price spectrum (fiocchi, rio royal, sellier & bellot) may throw larger patterns with some fliers due to the softer lead alloys used.
Best advice is to buy a small sample of several brands and pattern test them at ranges you would most likely encounter in defending your home.
Good luck, and have fun!