Which is the best question to ask someone who is providing feedback on your work?

The best questions are "What is your honest opinion?" and "What do you think needs the most work?"

However, when you ask someone these questions, expect hard criticism, and be willing to take it as an attempt to help, not as a personal attack on you. And most of all, don't give up when you get criticism. The only way to get better is to make your mistakes, and learn from them.