Relative pronoun से पहले आने वाले nouns/pronouns के अनुसार verb का प्रयोग होगा।
She is one of the noblest women that have ever lived on this earth.
(women के अनुसार verb का प्रयोग किया गया है क्योंकि यह that - relative pronoun के पास है।)
This is the book which was missing.
It is I who am helping them.
Shakespeare was one of the greatest dramatists that have ever lived.