I don't think many people would ever refer to margarine as "butter" - though they might talk about "buttering" a slice of bread when they meant using margarine. Just as we still do with all the weird and wonderful spreads that they make these days. I mean you couldn't very well ask for "I can't believe it nit buttered" toast could you?

Some people used to stretch the butter ration by mixing it with margarine, in which case "best butter" would be plain butter.

But generally I think "best butter" just means what it says, the better quality (and probably more expensive) butter that you might use for special occasions. Just what sort of better quality butter that would be would depend on family preferences. Just have a look at the range on sale in the shops, it's by no means all the same is it?