With a doubt, the one question that's done the most to make me a better programmer (and person) is "What will they say?" I wish I'd known about it when I first started working. Nothing makes you seem smart and prepared quite like having heard the question already and having the answer ready, and it's amazing how often you can predict those questions yourself.
About to get a Code Review? What will they say? Which design choices will they ask about? What won't they like? What tests will they ask if you've tried? Over time, I've caught more and more of my own issues and been ready to explain difficult decisions when the reviewer first arrives.
About to ask someone a question? What will they say?What details about the problem will they ask for? What debugging steps will they ask if you've tried? What will they suggest you look up? I can't believe how many problems I've solved while collecting the information I think someone else will need to figure it out.
About to demo something? What will they say?What details will they ask about? What is awkward that they'll want worked on a little more? What is too slow that you have to talk over? What's "hand-wavy" that they'll see is missing? What will they be most excited about? Nothing helps me to polish a design quite like trying to make a real-world demo completely smooth. Great applications make for great demos.
I’m sure there are endless examples, but you get the idea. How about you? What question has helped you the most?