Give the award to ____ you think will use it best.

In the clause “you think will use it best,” the word filling the blank functions as the subject of the verb “will use.” Even though the blank comes after the preposition 'to', which normally takes an object (whom or whomever), the grammatical role within the clause determines the choice. Since the pronoun acts as a subject inside the clause (“whoever will use it best”), “whoever” is correct. We use “whoever” instead of “who” because the word is part of a larger clause: “whoever will use it best”, which functions as the object of the preposition to, while inside that clause it also acts as the subject of will use. “Whoever” can play both roles at once (object of the outer clause and subject of the inner one), but “who” cannot.

So the full sentence reads: “Give the award to whoever you think will use it best.