German (and very likely other) speakers sometimes use the phrase at best incorrectly, and have no idea how big a blunder that is. The problem is that at best has the meaning of the best thing in a bad situation, and therefore has to be used with care. For example,
I’m at best qualified for the job.
suggests that you are not sure you are suitable for the job, that you think that you may be suitable, and that there are certainly people who are better qualified than you are. If you want to express that you are the best person for the job, you should say, e.g.,
I’m the best candidate for job.
I’m the best qualified candidate for the job.
The origin of the incorrect use of at best by German speakers is very likely the German expression am besten, which expresses that somebody or something is the best in a certain situation or context.