Hagrid
WARNING: Book 2 spoilers (duh) and small book three spoiler.
Ok, so Hagrid was innocent, as we all know. So then, why wasn't he allowed to become a wizard? I mean, they let him be a teacher, true, but he didn't commit the crime that got him expelled! You'd think Dumbldore'd do something to pay him back for all he went through, ya know?