"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.
5 In Europe, it is not uncommon to receive emails with the valediction With best/kind regards, instead of the more typical and shorter Best/Kind regards. When I see a colleague of mine writing such a phrase, I usually point out that it is a kind of old-fashioned affected valediction which, probably, nowadays, a native English speaker wouldn't ...