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. What was best? if we insert the word the, we get a noun phrase, the best.
It is also a superlative, like greatest, or highest, so just as you would use it as an adjective to show that something is.
In the following sentence, however, best is an adjective: This is very good instinct, and you could. You could certainly declare that after. 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.
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.
