Generaly speaking the smaller the shot the more area it is going to damage. However the smaller the shot the less penetration each pellet is going to have. So the ideal shot is the one that goes deep enough to do enough damage, yet is as small as possible to accomplish that so you maximize what I call the "hamburger" effect. Big pellets make holes, tons of pellets make hamburger.
Distance and potential cover change the criteria a lot. Patrol officers often use (per department policy) 00 because it is effective out to a longer range, and they need some penetration of cover as well. So they pick a general purpose round that does not come up too short on any specific area.
CQB guys often use #4 buck because it is really nasty and has a high hamburger effect and in the real world often penetrates deep enough. Some feel penetration needs to be at least 12" deep to reliably hit organs from all angles. Usualy they won't be that deep, but to accomplish that #1 buck is the best load.
The idea is to keep the pellet count and "hamburger" effect as high as possible and still penetrate as deep as necessary. Bird shot has the most massive hamburger effect, but will not penetrate deep enough to destroy organs reliably. This means tissue that is hit with birdshot is pretty much destroyed at close range, but the wound is only a few inches deep and while the tissue will be beyond repair, it won't surely stop the guy from continuing an attack if determined.
The reverse is that the bigger you get, the more of the shotgun hamburger effect you lose, which is what really makes them so deadly. Having something important in the body turned into instant hamburger is going to have a much more immediate effect than simply putting a hole through it, but it has to get deep enough to reach it.
Some are firm believers in #4 buck, which seems to be more common for sale than #1, and others don't want something that penetrates less than 12" and consider #1 buck the best overall by having the highest pellet count that can also penetrate 12" of ballistic gelatin. Yet others have barrier considerations, walls, car doors, windshields, longer distances to contend with and need 00,000 or slugs to both penetrate that mild cover and still penetrate enough to be effective or still be effective dozens of yards away.
If #1 is best directly on the target, 0 or 00 would be necessary for the same penetration after going through light cover.
Police use 00 because it covers the biggest spectrum of uses, not because it is necessarily the best one shot performer. Coincidentaly this has caused most people to feel it is the most adequate for thier defensive needs because if the police rely on it, it must be better suited for thier needs. So 00 is often the cheapest defensive caliber and the one you will find the best deals on and be able to practice with the most because it has the greatest demand and therefore the greatest production.
Also consider most ballistic tests are conducted at a range much greater than across the room distances, so the penetration factor is higher than rated. However tissue is not all uniformly the same density either, so it tends to balance out.
Further criteria than simply pellet mass/size and therefore momentum can also cause slight variations. Plated, hardened, pellets will penetrate deeper than soft lead ones. So for example, hardened plated #4 penetrates as deep as a slightly larger buckshot size because it deforms less and makes better use of its energy. Bringing it closer to #1 which meets the 12" criteria.
However "tactical" or "reduced recoil" loads will either have a reduced payload, or reduced muzzle velocity which means they will penetrate more like a lesser buckshot size. So if someone likes such rounds they would want to go to slightly larger pellets than what they feel is necessary to keep the same level of penetration.
So there really is no uniform perfect answer for all situations. Pick what suits you.
