The pheasant would be the only bird (if wild pheasant) that would even seem worthy of a 12g shell, IMO.
What exactly is your criteria for this statement??. This surely would not be touching upon that above mentioned "Elitest" attitude that a 12ga is like unto shooting ducks in a rainbarrel While shooting a smaller gauge is the sign of a "Real Man & Suberb Sportsman" is it.
Basically I really can't think of any other reason to make such a statement. I have a 12ga sidelock 28" damascus, wt 6lb 14oz which I found a total joy for upland use loaded with 1oz shot to about 1100-1150 fps. I have used this gun to take quail, woodcock & rabbit & found them all quite worthy of the gun. Could i have made most of the shots I made with it, with a smaller gauge, Undoubtably. But I simply liked that gun & enjoyed shooting it. Would a smaller gauge have been more fun or made me more of a "Man", Absolutely Not. I once killed a goose with a 20ga & some others with a 10 mag. Those taken with the 10 were "More Worthy".
A lot more factors than size of the hole enter into any equation, but when the question says "Best All around" the answer is still 12ga.
Maybe this wasn't really what he meant, but that's what he asked.
