Having just returned, I would have to say May (third time for that month for me). It stays light late, park hours are long, crowds are medium, it's pool weather and shorts are suitable in the evening. And I don't have to return home to snow like in the winter months, which makes the last day at WDW much more bearable.
Halloween. Not as painfully hot, 95% of the lines are under five minutes, (Peter Pan was still at least ten) you get to dress up, you get to hang at the Kingdom 'till Midnight, and of course, FREE CANDY!!!
Second would go to mid-late November when all the Christmas stuff is up and going, but it's not nearly as crowded as actual Christmas time