I mean here "You are the best at tennis" "and "you are best at tennis", "choose the book you like the best or best" both of them can have different meanings but "most" and another adverb in a standalone sentence has a completely different meaning.
So, " It is the best ever " means it's the best of all time, up to the present. " It was the best ever " means either it was the best up to that point in time, and a better one may have happened since then, or it includes up to the present.

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The second sentence, as you said, contains a superlative, "best." In English, unlike in some other languages such as Spanish, the superlative does not require a definite article. In fact, it would actually sound weird to say, "It is the best to stay here." The second sentence means this: It is better to stay here than anywhere else.

The word "best" is an adjective, and adjectives do not take articles by themselves. Because the noun car is modified by the superlative adjective best, and because this makes the noun car definite in this context, we use the. It is best not to do something. Here, we have the adjective best, but this adjective is attached to no noun.

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Your example already shows how to use "best" as an adverb. It is also a superlative, like "greatest", or "highest", so just as you would use it as an adjective to show that something is the ultimate example of it's kind when used as an adverb you do so to indicate that the adjective it precedes is to the highest degree possible. In your example "experienced" is the past tense of the verb to ...
"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. This is very good instinct, and you could even argue that the grammar is good, but at best it's unnatural.