The word best is an adjective, and adjectives do not take articles by themselves. 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:
Your example already shows how to use best as an adverb.
This is very good instinct, and you could. What was best? if we insert the word the, we get a noun phrase, the best. 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.
Because the noun car is modified by the superlative adjective best, and because this makes. 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.
