Pick a skill set that is an impressive skill set, not required for the job but related to the job that you do and say that your weakness because you've recently started learning it.

I'm a data analyst. Programming is not needed but a little bit of scripting knowledge is nice. So I used to tell people my weakness was programming. Now that I've gotten pretty good at programming I'll probably say something like software engineering and caveat that I'm pretty good at programming but I'm not quite a software engineer yet. When that is no longer applicable I'll probably say something like systems architecture.

You just keep moving the ball to a more and more advanced skill that makes you look good, is relevant to your job, but not a required skill.