Margaret
All analysis or deep theological ventures aside, and from a place of pure entertainment, I really have to say that Margaret Chenowith was one of the most fantastic characters in this series.
She obviously may not have had the exploration and depth as any of the main characters, but if Joanna Cassidy didn't absolutely steal every single scene she was in. Amongst the pantheon of masterful performances and nuanced characters in the series, I feel as though Margaret and Cassidy seem to get overlooked, but she really was one of the highlights of the series. Almost every word out of her narcissistic and self-obsessed mouth makes me laugh.
That being said, one of my favorite scenes of the series is the scene she and Brenda shared in the Season 5 premiere, A Coat of White Primer, when they discuss Brenda's miscarriage. While to be fair, it does seem somewhat out of place, I think that relationship growth between the two of them was subtle but nevertheless present over the course of the series. This scene showcased a nurturing and bizarrely maternal side to Margaret that we rarely ever were allowed to see.
Brenda of course talked a big game about detesting her mother and being endlessly resentful of her upbringing, which she clearly was, but I do think there was some reluctant admiration there. Not that she'd ever admit to it, but Brenda obviously cared greatly about the opinions of her parents, trying endlessly to prove that she wasn't like her mother, when in reality they were both painfully similar. They both knew each other so well, Brenda was so begrudgingly in-tune with her mother's motives and behavior, that she spent every day trying to convince herself that they were nothing alike.
While nobody would argue that she was anything close to the mother of the year, there was little doubt in my mind that she loved and adored the both of her children in a very warped and lab-tested way. Brenda and Billy were undoubtably Chenowiths, too self-aware and too intelligent to ever let themselves be happy, and oddly enough I always saw a proud Margaret in that regard.
And of course, you can't beat the Tranquility Spa scene.