I have interviewed and hired hundreds of people, and this question is critical. The specific answer is almost utterly useless. What is critical, is the self-reflection and honesty in the answer. I promise that honest answers, especially if they demonstrate some keen self-awareness insight, are immensely more important than the topic of the weakness. Any of these psuedo-weaknesses (perfectionist, optimism, workaholic, etc.) are clearly seen at best as trite dismissive answers to the question, and at worse just outright lies. Answers such as Xaelias above, show a deep understanding of oneself, and attempts at iself-mproivement. In all the interviews I have conducted (over 1000), such an answer would always lead to a positive impression of the candidate, not a negative. We all of real weaknesses (well maybe not Dr. Kevin40 😉 ), and the sooner we can all realize ours, the sooner we can rid ourselves of them.
I'll contribute mine...just to take my own medicine. I am terribly self-critical and lack self-confidence. It leads me to over dependent on others opinion of me, my actions, and my decisions.