My favorite question is "What would be your positioning statement for yourself as a product marketer?"
I love that question because it shows me how comfortable they are with the basics of positioning, and it tells me a lot about the aspects of product marketing that they care most about. For example, they may highlight their strength in messaging and positioning, or their customer-centricity, or their ability to partner with teams across the company. It's also another opportunity to see how they communciate complex concepts in a clear, concise, and compelling way.
I've heard a lot of great responses, and there is no right answer. But I always come away with a deeper understanding of how they view their strengths, as well as the type of work they like to do.