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

Solution 1:

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."

Solution 2:

When using a singular phrase like 'one of the best films', 'each person in the group', 'every student in the class' the verb should be in the third person singular form. 'One of the best films that has ever been made'; 'Each person in the group is responsible for his work.'; 'Every student in the class is an adult.'

I think modern writing would strike out the connector 'that has ever been made' as so much excess verbiage and let the sentence read 'It's one of the best films ever made.' Much simpler, less pompous and certainly more common in usage than the wordier version.