There's lots of advice out there on how to handle interview questions. There are resources about the questions themselves, particularly the tricky ones like how many ping pong balls can you fit in a 747.
What is your greatest weakness (or something like that) has become a staple question, and, as suggested here, the facile answer does little to set you apart; perhaps it even limits your chances.
Being self-aware gives you strength (you know what to work on) and gives you a viable answer to this question that demonstrates you know you are not perfect and you're working on it. For me, my greatest weakness is confrontation. What's your weakness?
The fact is, it’s not easy to identify one’s own weaknesses. Doing so takes intense self-reflection, critical thinking, and the ability to accept negative feedback–qualities that have gone severely missing in a world that promotes instant gratification and demands quick (often thoughtless) replies to serious issues.