Your example already shows how to use best as an adverb. What was best? if we insert the word the, we get a noun phrase, the best. In the following sentence, however, best is an adjective:
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Best wishes 和 best regards 有什么区别?首先我们来看下best wishes和best regards的大致意思：best wishes：词性为短语，用于表示对某人的最好祝愿，通常用于书信、贺卡等正式场合.
This is very good instinct, and you could.
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. You could certainly declare that after.
