Which sentence is grammatically correct?

The sentence "It was I who won" is grammatically correct because it uses the subjective case pronoun "I" after the verb "was." In English, the verb "to be" (including "was") links the subject of the clause to a subject complement. In this case, "I" serves as the subject complement, and it must be in the subjective case. Although casual speech often uses "me" in similar constructions, using "I" is the grammatically appropriate choice in formal writing and contexts.

This adherence to grammatical rules highlights the distinction between subject and object pronouns, which is crucial in formal English grammar. The use of "I" following "was" maintains proper subject-verb agreement and clearly identifies the subject performing the action of winning.