To ask a winning question, think about your audience, shareability, and having some edge.
Make sure you're asking a question to the right leader. Read their bios and see what their stance is on issues you care about. The more difficult your question, the more fun things will be.
Your question will never make it into the top three if other people don't like it! Make sure your question is highly shareable - there's only so much campaigning you can do yourself. Keep it short, snappy and biting.
Having some edge
Try to avoid long, boring questions. If your question is attention-grabbing there's more chance you'll get picked up by media and blogs. Remember, only the first 100 characters or so will appear in tweets, Facebook shares and so on.