My favorite kind of interview question/challenge is anything that is highly practical for the job. At the current company I work, the coding test/interview challenge was to design and implement an API very similar to the core functionality of the actual product. It’s fair, tests for skills relevant to the job, and is much better than irrelevant silly brain teasers and cs questions, I feel.

In terms of specific questions, one of my favorites is one that one of my colleagues suggested I ask to potential candidates: describe what you think your biggest failed project/task was in your engineering career, and what happened/what you learned. I think it’s a good reflective question that can tell a lot about someone.