"Which one is the best" vs. "which one the best is"

Question

I am writing a paragraph like this:

There are mainly three ways to check which type a given value is. In this post, I am going to cover what they are, when to use them, and, in my opinion, which one is the best?

I am not clear on the last bit of the sentence, "which one is the best". I thought it is similar to the clause in "I am going to cover what they are", the order of object and the verb is reversed (I forgot what the grammatical term is), "which one is the best" should instead be something like "which one the best is".

My question is, what is the correct way to write this clause?

Answer 1

Your original is correct as-is, except you need to remove the question mark at the end because it's not a question.

What I imagine you are already thinking:
The sentence ends with a string of "wh-" noun clauses. These clauses are not questions, so the last one should also not be a question. "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.

When we replace the superlative "the best" with the normal version, we get this:

*I am going to cover which one good is.

I hope we can both agree this sentence is wrong because "good" is an adjective, and cannot be the subject of "is". The same is true when you put "the best" in that spot. It's easy to argue that "the best" could be correct as the subject since superlatives are often used as nouns (as in "The best is this one" where "the best" stands for "the best one"), but when reworded into a noun clause, superlatives only feel like adjectives, so "which one the best is" doesn't work.

Now, you might be thinking that "which one is the best" cannot be correct since it's a question format. The answer is that like many other similar phrases, the question format and the noun clause format look the same:

Which hammers are on sale? (question format)
I'm hoping you can tell me which hammers are on sale. (non-question format)
*I'm hoping you can tell me which hammers on sale are. (bad grammar)

For a more thorough explanation of why the two formats look the same, see JavaLatte's answer and note that "the best" is a complement.

Answer 2

When the subject and the auxiliary verb are swapped over, it's called inversion.

A question word can function as subject, object, complement or adverbial. When it is the subject, inversion does not take place.

Who knows the answer? -subject- no inversion
Who did you ask? object- inversion
Whose book is this? complement- inversion
Why didn't you go to the party? - adverbial- inversion

In your sentence, which is the subject, so no inversion takes place. In addition, the final clause is a statement, not a question: which is functioning as a relative pronoun, not as a question word. The same would apply if it were an adverbial. Compare these two sentences:

I don't know why he did that. - statement- no inversion
Why did he do that? - question- inversion

For more information about this, see section 25 of the Oxford Guide to English Grammar (John Eastwood, 1994)

Answer 3

"which one is the best" is perfectly fine, "which one the best is" is wrong.

Examples of "which":

Which car are we going in?

Which museums did you visit?

Which do you prefer?

In the Young Cook of Britain competition, the finalists were asked which famous person they would like to cook for.

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