0 The phrase 'one of the' is used to describe something/someone from the same group. There are many birds on the tree. One of the birds is red. This means we are talking about all the birds on the three but then when you want to be specific about the red bird, you use one of the. He is one of the soldiers who fights for their country - is correct.
"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.
Some people say a dog=one, dogs=ones, the dog=the one=that, and the dogs=the ones=those. It's a rule of thumb, but what I found was that this is not always correct.
How one and one's is different from other indefinite pronouns The possessive of one (one's) is formed the same way as the possessive of other indefinite pronouns, such as someone (someone's), but it is used a bit differently. For most people, one is consistently used with the possessive form one's.
One-to-one is used when you talk about transfer or communications. 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. See Free ...