Even more so than the question about strengths, it’s unusual to be asked about your weaknesses. There is no good way to answer this question so the best advice is to try to move on as quickly as possible. Obviously you don’t want mention real weaknesses (I’m dumb, I’m lazy, I require 12 hours of sleep a night). You also don’t want to say things that make you look silly like “I work too hard” and you can’t say you don’t have any weaknesses because you’ll come off as too arrogant. So try to think of something relatively innocuous that also might highlight a strength. For example, “I can get occasionally get impatient with peers/coworkers who don’t have the same abilities as me or don’t show the same commitment that I do.” Or, “Sometimes I can be so focused with or driven by the task on hand that I wind up tuning out other aspects of my life.” You can also usually say something like, “I think my skills are very good compared with my peers but, of course, I’m new to investment banking, and I obviously need more experience. Experience which I’m confident I’ll get working for you…”
Occasionally, a really difficult interviewer will ask you for 3 weaknesses, knowing that your first 2 will be bullshit answers. To which I would respond that my major weakness is, “I’m really bad at bullshit interview questions.”