For defense purposes against a human aggressor, the answer gets a little muddy. While it is always good to put as many pellets as possible on target, other factors come into play when a shot is made in a populated area. You are legally, ethically, and morally responsible for any projectile(s) you send down range. Knowing how your load patterns at specific ranges is critical. Many LEOs are now required to carry only low velocity buckshot loads as the lower pressure and buffered shot results in tighter patterns and better shooting due to reduced recoil, while other departments have dropped the shotgun altogether in favor of .223 or 9mm auto carbines. The difficulty with the very tight pattern from the LEO buckshot loads is that rifle type marksmanship is required, as these shells can produce head size patterns at 25 yards. A solid hit is a decisive hit, but a miss is a complete miss.
If defense is wildlife related, I opt for the 00 load my gun will pattern the best and put the most pellets on target, which in the case of my 590 is Federal's 3" Premium 00 Buck load and I keep a few of these in my Side Saddle to compliment my Brenneke slug loads. I can keep all the pellets from this load on target at 25 yards. But every shotgun barrel is a law unto itself, so don't assume that Federal Premium is best for your gun; buy a variety of ammo, and see which patterns the best for you from 5 yards out to 30.