I would say what is best to ask depends on (i) what your interests are; (ii) who is interviewing you; and (iii) what has previously been discussed in the interview:
- Firstly, you want to ask something that you actually care about. Whoever is interviewing you likely has significant experience with the process and will therefore be able to see whether you are being genuine or just asking a question for the sake of it. Thus, when you think of a potential question, first ask yourself why do you think the answer might matter to you. In fact, I think it would be good if in the interview you mention the reason why you care about the question before you actually ask it - this will directly show the interview that you have put thought into it. An example of questions that you may choose to ask could be questions relating to your 'Why the firm' motivations. For instance, if a a strong reputation in one practice area, you may ask: how has the firm managed to build such a strong practice? what are plans for the future of the practice? are there any relevant challenges and opportunities in that practice's market, and how is the firm planning to respond to that? how can the firm's practice be differentiated from competitors with similar reputations? etc.
- Secondly, you want to tailor your question to the audience as much as possible. Some questions may be more appropriate for an associate, some for a junior partner, and some for a senior partner. For instance, questions relating to junior culture and training experience are great for associates, questions around progression and the different tasks and responsibilities at different seniority levels in the firm would be great for a junior partner, while questions about the firm's strategy, client base, and market reputation when compared to competitors would be great for the more senior partners. Moreover, you may not want to ask a disputes partner about the firm's strategy to increase market share in PE, and you may want to avoid asking a transactional partner about details of the firm's newest competition litigation mandate. If possible, tailoring your questions around your interviewer's expertise is optimal.
- Thirdly, you want for your questions to feel natural - you don't want to seem like you came in with a pre-prepared list which you were going to ask regardless of how the interview went. Of course, it is good to show that you have done your research and came prepared. However, you also want to make this section of the interview feel as conversational as possible, as for once you are playing the role of the person doing the asking. The more you can link your questions with what has been touched upon in the interview, the better. Some questions could just be follow-ups to the interviewer's answers to your pre-prepared questions. Some could be just about asking the interviewer to elaborate on some points they touched upon beforehand. However, take care to not be repetitive. Thus, only ask a question if you think it is likely there is a lot more that the interviewer could have to add to the previous discussion.