best
The highest comparative form (superlative) of the adjective 'gut' (good); expresses the highest quality or excellence.
Das war der beste Film, den ich je gesehen habe.
That was the best film I have ever seen.

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The highest comparative form (superlative) of the adjective 'gut' (good); expresses the highest quality or excellence.
Das war der beste Film, den ich je gesehen habe.
That was the best film I have ever seen.

You have to distinguish between the form before a noun ('der beste Wein' — the best wine) and the adverbial or predicative form ('am besten' — best). A typical mistake is using 'am besten' before a noun.
Das ist der beste Kuchen, den ich je gegessen habe.
This is the best cake I have ever eaten.
To give advice, 'Am besten' (it would be best) can be placed at the beginning of the sentence. The verb then follows directly in position two.
Am besten, wir treffen uns direkt am Bahnhof.
It would be best if we met directly at the train station.
In the fixed expression 'sein Bestes geben' (to do one's best), the possessive pronoun adapts to the person who is making the effort. So it is 'ich gebe mein Bestes' (I do my best), 'du gibst dein Bestes' (you do your best), and so on.
Ich weiß, dass du dein Bestes geben wirst.
I know that you will do your best.
good
'gut' and 'best' are closely related because 'best' is the superlative form of 'gut'. With 'gut', you say that something is positive, correct, or of high quality. With 'best' or 'am besten', you compare several options and say that one of them has the highest quality or is the most suitable. Learners may confuse the words because both sound positive. The important point is: 'gut' is enough without a comparison, while 'best' usually needs a comparison or a choice.

Von allen Terminen passt mir der Freitag am besten.
Of all the dates, Friday works best for me.
Der neue Plan ist gut und hilft dem Team bei der Arbeit.
The new plan is good and helps the team with their work.