Which of these sentences is correct: "If I was you, I would do it" or "If I were you, I would do it"?

The correct sentence is "If I were you, I would do it"." This is the subjunctive form of English. It should be used when a hypothetical is contrary to fact. In this case, "I" is not "you," so the subjunctive form should be used. In first person, the form of "be" used for the subjunctive mood is "were."