Anjelica Huston as The Grand High Witch in Nicholas Roeg's The Witches(1990).
Horrornews.net has my article up listing the 10 best movie witches of all time. Looking it over now, however, I realize that I completely slighted Joan Bennett as Madame Blanc in Suspiria (1977). (I haven't seen the remake).
It's still a good list, however. Check it out.