Good thread, and I wonder the same as I'm thinking a few years in the future when we (hopefully) will return.
We are Canadian and from crowds and cost, the last week in August would be the best. But I don't think we could stand the heat (well, I love heat, but DH would hate it and I would worry too about my kids overheating). Also, I've heard there is lots of rain so that sounds miserable (although maybe is a good thing to cool off). Same issue in July in terms of the weather.
Christmas, everyone is off the same time so I would think that would be the worst time.
So I voted March Break...but NOT Easter so assuming these two do not coincide. (I"m actually tracking crowds on the Disney app this year to get an idea). At least the crowds will be spread out depending on the week of the break (although I think in Canada we're all the same) and weather should be nice. Downside as well though is the cost, both Disney and airlines.
PS - We don't have a mid winter break here, but if we did, February sounds like a good option too.