"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. One-to-one is used when you talk about transfer or communications.

Understanding the Context

You may use one-to-one when you can identify a source and a destination. For eg., a one-to-one email is one sent from a single person to another, i.e., no ccs or bccs. In maths, a one-to-one mapping maps one element of a set to a unique element in a target set. One-on-one is the correct adjective in your example.

Key Insights

See Free ... With one or more is / are, the first thing to consider is whether 'one or more' is a unit or analysable. It has the near-synonym 'some'; 'four or five' could be substituted reasonably by 'several'. Which is correct: "one or more is" or "one or more are"? Regarding one hundred or a hundred etc, the person saying that there is a difference is right - one is used more for precision but a is more common and employed.

Final Thoughts

Which one is grammatically correct or better? I have two assignments, One of them is done. I have two assignments, One of which is done. I watched a video tutorial that the teacher said the ... pronouns - "One of them" vs. "One of which" - English Language Learners ...

one-on-one is used to talk about meetings between two people. When there is a discussion we can call it a one-on-one discussion; as an alternative for a face to face confrontation and in interviews (quite often political ones on TV).