I love that you're asking about this. It's such a great opportunity for you in an interview. While I like the spirit of the "I'm a perfectionist" answer -- because it flips the answer -- it's such a common answer that it sounds rehearsed and perhaps not honest. It's so common that a lot of interviewers will say "Tell us about a flaw you have, and you can't say that you're a perfectionist or that you work too hard."
Use the "worst flaw" question as an opportunity to show how you learn from mistakes and you grow. Also, that you're actively engaged with self-improvement.
- Pick something that you're already working on and getting better at. Be honest. If you're not working on getting better at something in your life, pick a bad habit and start making even small steps to improve it now. Then, that can honestly become part of your answer. For example, maybe sometimes you rush right into doing something without making a plan. But only pick this if it's something you're already working on.
Example A: "I like this question, because I'm always looking for ways to improve. Right now I've been working on my tendency to jump right into doing something when sometimes I should take a step back and make plan."
Example B: "In school, over the last year, I've been working on not waiting until the last minute. I feel like I'm good under pressure and there's some part of me that actually likes pulling an all-nighter to get a paper done, but I realize it's not helping me in the long run. So, I'm working on starting things earlier and breaking them up into manageable chunks.
- Describe how you're already improving. This is huge. It shows that you're reflective and that you have a growth mindset; two qualities that ANY good manager would want from you.
Example A, continued: "I'm making progress. A year ago, when I was [insert something from some previous job or how you approached something at school], I was excited to just get into it. Halfway through I realized I'd spent too much time on xxx and not enough time on yyy."
- Tie it back to the job that you're interviewing for by showing how you'll continue to improve, if possible.
Example A, continued: "In this role, I'll continue improving on this by reviewing the weekly schedule and reflecting on what needs to be prepared ahead of time and then at the start of each day, I'll review..." <-- That's just an example, of course.
- Alternate way to tie it into the new role: Show that you'll work with your manager and peers to improve, if it's not yet obvious how. If it's your first job, or maybe it's not clear how you'll actually continue working on this, you can demonstrate another great character trait: willingness to learn from others and communicate well.
Example A, continued: "I'm sure there will be ways for me to continue improving while I'm in this role, and I'm excited to learn more specifics of the day-to-day tasks. Especially in the beginning, I'll reach out to my colleagues and my manager to see if they have ideas and feedback on how I'm doing to improve my habit of making a smart plan." <-- or whatever your issue is.
I hope that helps.