I agree that "When You Wish Upon A Star" is THE Disney song. Disney has it as a theme song signalling the start of all their movies as well. All those other Disney songs have had their big moments in time when they were THE Disney song everyone was singing and they all can instantly be recognized as Disney. But I'd have to say #2 Disney is Zip A Dee Doo Dah.
Either way, it looks like the best bets for #2 are "Great Big Beautiful Tomorrow", "Zip-A-Dee-Do-Da", "Its A Small World" and "Mickey Mouse Club Song". The final three almost everyone knows despite never setting foot in a Disney park. The first one I think you have to have been to Magic Kingdom to know it. So I would lean on the final three as the best options.