Here's why:
* Noun clauses act as nouns. They can be the subject, object, or complement of a verb. In this sentence, the entire clause acts as the object of the preposition "to".
* "Whomever" is the subject of the clause and "did the best job" is the predicate.
* The entire clause functions as a unit, providing information about the recipient of the award.
Let me know if you'd like more examples or explanations!