Season 13: Diane Pierce
The arc with Maggie's mother had everyone in shambles of course. After her divorce, Diane (LaTanya Richardson Jackson) learns from Jackson that she has breast cancer. When Maggie finally learns about the diagnosis, she finds every possible treatment for her mother, who eventually passes away on her own terms. Diane's story really gives us a look into Maggie's life and history outside of her sister drama with the Grey's and it's both sweet and heartbreaking.