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.
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.
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.