Whatever will knock the steel over and cycle your shotgun, if you're using a semiauto. Only very rarely would you need buckshot. For slugs, the low recoil slugs are a nice way to cut down on your aspirin consumption.
I use a Remington 1100 and it needs 1 1/8 ounce loads to be reliable.
Some shooters use "spreader" loads for some courses of fire, and I'm thinking of trying that.