Granny Weatherwax
She has recruited at least one apprentice in her time, Eskarina Smith, who became the Discworld's first female wizard ("Equal Rites") and was subsequently not mentioned in canon again until the events of "I Shall Wear Midnight". Magrat Garlick, Agnes Nitt and Tiffany Aching were officially taught by other Witches, but also learned from Granny Weatherwax from time to time.