grammatical number - It's one of the best [plural noun] that + has (or) have

In the sentence

It's one of the best films that has/have ever been made

What should the verb agreement be?

Answer

It's plural "have".

A useful way to prove that a plural verb is correct is to front the preposition phrase:

"Of the best films that have ever been made, it's one of them."

NOT "Of the best films that has ever been made, it's one of them."