Definition of “best” - Learner’s Dictionary

best

noun uk us /best/
the best

B1 someone or something that is better than any other:

He's the best of the new players.
at best

used to show that the most positive way of considering something is still not good:

At best, only 50 per cent of babies born at 24 weeks will survive.
at his/its, etc best

B1 at the highest level of achievement or quality:

The article is an example of journalism at its best.
do/try your best

B1 to make the greatest effort possible:

I did my best to persuade him.
bring out the best in sb

to cause someone's best qualities to show

make the best of sth

B2 to try to be positive about a situation you do not like but cannot change:

Our hotel room is rather small, but we'll just have to make the best of it.
for the best

If something is for the best, it seems unpleasant now, but will improve a situation in the future:

Divorce is always painful, but it really was for the best.
at the best of times

used to show that something is not good when it is the best it can be:

He's not exactly patient at the best of times.

(Definition of “best noun” from the Cambridge Learner's Dictionary © Cambridge University Press)