Relative Pronoun after Superlative Degree
Incorrect : She is the best singer whom I have ever seen.
Correct : She is the best singer (that) I have ever seen.
Incorrect . This is the best book which he has ever written.
Correct : This is the best book (that) he has ever written.
Note: The relative pronouns who, whom and which cannot be used after the superlative degree. You can use the relative pronoun that after a superlative. But in most cases this relative pronoun is omitted.