"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. With one or more is / are, the first thing to consider is whether 'one or more' is a unit or analysable.

Understanding the Context

It has the near-synonym 'some'; 'four or five' could be substituted reasonably by 'several'.