You could certainly declare that after. Your example already shows how to use best as an adverb. This is very good instinct, and you could.
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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.
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. 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.