You use at best to indicate that even if you describe something as favourably as possible or if it performs as well as it possibly can, it is still not very good.
This policy, they say, is at best confused and at worst non-existent.
At best they were effective as antidepressants for no more than four months.
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at best in British English
a.
in the most favourable interpretation
b.
under the most favourable conditions
See full dictionary entry for best
at best in American English
1.
under the most favorable conditions or interpretation
2.
at most
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Examples of 'at best' in a sentence
at best
Truth, in his book, seems to constitute at worst an admission of defeat, at best a loss of face.
Gagman, Maurice DOUBTFUL MOTIVES (2003)
You know that our relations with Chalced are at best uneasy.
Robin Hobb THE GOLDEN FOOL: BOOK TWO OF THE TAWNY MAN (2003)
Mercer said the Feds know their case against you is circumstantial at best.