Because the noun car is modified by the. 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.
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The word best is an adjective, and adjectives do not take articles by themselves. 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. I like chocolate best, better than anything else can be used when what.
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.
Both sentences could mean the same thing, however i like you best.
