It depends on what you value most in a working relationship. What I want in a boss isn't necessarily what you want in a boss. General questions that might apply to most people would include things like "What kind of working hours do people on your team keep", "How do you help build the careers of people on your team", "Why did you choose this project and what do you find exciting about it", "If you were to start this project all over again knowing what you know now, what decisions would you make differently", "How are relations between the tech side of the team and the business/product side of the team", "Why should I join your team in particular", "How well do the members of the team get along and what does a typical day look like -- do people sit together and chat frequently? Not interact in person much?". You see where I'm going with this.