#1, #B, and #BB here. I'll explain.
If I'm hunting rivers/marshes for ducks with the chance of a passing goose close enough to shoot, I'm packing #1's or #B's depending on if it is just mallards or a mix of mediums/larges (gadwall, wigeon, ringnecks, mallards).
If I am hunting field for geese with the chance of ducks coming in, I'm shooting #BB.
I have yet to hunt a place where the chances at ducks and geese were equal, so I seem to lean towards one or the other.
For just duck killing you can't beat #2's, but where I hunt the geese won't commit to our duck spreads so our shots are usually 35-45 yards overhead. #2's are crippling machines on those types of shots. Our geese here are ALL giants. About 12lb average on them. If you are shooting the regulars or lessers, #2 shot might be the ticket! But I have yet to see a load of #2 steel reliably kill giant canadas unless they were decoying in and committing suicide. And by kill I mean kill, not knock them down and chase them across a marsh, or sail them for 75 yards before they bleed out.
This season I'm trying out a new combo load, 3.5" 1 1/2 #1's. Should be enough pellets to reliably kill a duck at any reasonable yardage and yet still have enough power to take those overhead geese. We'll see how it does.
Why do I shoot 3.5" for geese? Because they don't make a 4" yet!