In the following sentence, however, best is an adjective: What was best? if we insert the word the, we get a noun phrase, the best. Best wishes to you：我真诚的祝福你，祝福您； best wishes to：给。。。。最好的祝福； 例句:please convey my best wishes to him.
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Your example already shows how to use best as an adverb.
Which one is the best is obviously a question format, so it makes sense that which one the best is should be the correct form.
It is also a superlative, like greatest, or highest, so just as you would use it as an adjective to show that something is. This is very good instinct, and you could.
