Hey!
I would treat this as a motivation + self-awareness question, rather than trying to name a "perfect" firm. In practice, I'd avoid saying that you have a single "dream firm" unless it is the firm you are interviewing with. Instead, I'd frame your answer around what your ideal firm looks like, and then link it back to them.
For example, you could say that you are looking for a firm with a strong international platform, high-quality work in X area, and a collaborative culture - then, I'd explain why that aligns with what you have learned about the firm. It also helps to back up any points that you make with evidence from your past experiences or future career goals (e.g. if you say you are interested in working in X area, back it up with your experience from studying it at university, etc.). If they push you to be more direct (which I think is unlikely!), it is fine to say that the firm you are interviewing with is your top choice based on your research and experiences. However, I'd keep this grounded in evidence and avoid saying anything that sounds overly rehearsed or unrealistic (as no firm is perfect!).
The key is to show that you have thought seriously about what you want, your criteria are commercially aware and realistic (e.g. go beyond "working with nice people"), and that there is a clear and genuine link to the firm you are interviewing at. It is less about naming a "dream" firm, and more about demonstrating why you are a good fit for this firm (based on clear reasoning).
Best of luck with the interview!
Wishing you luck!
