This is a tough one for me - most of my favorite books featuring youthful protagonists have missing mothers - To Kill a Mockingbird, Treasure Island, Oliver Twist, for example.
Mrs. Weasley is a good choice as she has many of the classical good features of a mother - caring, interested, multi-faceted. However, I didn't read all the books - just saw the movies.
A "left-field" choice is Mrs. Quimby from the popular Ramona series by Bevely Cleary.
Looking forward to reading your choices and your comments on mine!