What's the weakest thing about you as an employee?
Maybe its proof of some knowledge or experience that's missing from your CV, maybe something about your personality which doesn't do you any favours.
Whatever it is, you should think seriously about picking off your weaknesses one by one. OK, I admit, some will be easier than others, and some will take more time than others (changing your personality can be hard!) But if you put your mind and some time to it, you overcome the weaknesses that might be stopping you getting that promotion or that perfect new job.