The thought hit me like a bolt of lightning. I'd say Ellen Degeneres, but she's the wrong physical type. Another possibility would be Patricia Routledge (Hyacinth Bucket from "Keeping Up Appearances"), but she's a bit old for the part.
Of course, the best person for it isn't an actress, although she'd come to the part with her own flying broomstick. I'm speaking, of course, of Lord Hildamort.