What is your favorite question to ask in job interviews?

I don't have a favorite, because no single question is very useful all by itself. My interview style also tends to be conversational rather than Q-and-A, so the questions I ask tend to vary according to the candidate.

But two questions that I nearly always ask are: tell me about a project you did (personal or professional) that you found very exciting, with a followup question of "what about it got you excited?"; and tell me about a project you did (personal or professional) that went sideways. This one often has a followup question of "how did you deal with it?"

Neither question has a right or wrong answer. Well, I fib a little. A wrong answer to the "what went sideways" question is "I can't think of one". The only way that can actually be true is if the candidate is very inexperienced.