it was the best ever means either it was the best up to that. Because the noun car is modified by the. 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.
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So, it is the best ever means it's the best of all time, up to the present. I like chocolate best, better than anything else can be used when what. Both sentences could mean the same thing, however i like you best.
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.
The word best is an adjective, and adjectives do not take articles by themselves.
