I usually stay honest. The main objective for me when I seek a job is to see if I will like it. I know my weaknesses, and they are in two categories: some are weaknesses I would like to improve and others I don't really care.
As an example, I am "okay" in math, and I do try to improve by watching Khan Academy videos and trying some exercises on the Internet (Laplace Transform, Fourier series, stuff like that). If the job I seek requires me to be good in math, I will explain it that way. Now, I am really bad at CSS, and I wouldn't seek a job requiring it.
I like what Rémi Mercier said about the "reverse situation" where the interviewers ask the company what are their weaknesses, I will keep that in mind.
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