Redskinluver:
Willow spanking Cordelia
Anyone spanking Cordelia (or another one of the popular girls) would have been wonderful.
Seegee:
Dawn was the one who most needed a spanking
Buffy and Willow seemed very spankable during the first few seasons. There was one episode - "Ted" - where a robot with a 1950s mentality entered the picture and I hoped it would lead to spanking.
Either of Buffy or her mother. Alas, Ted was just too brutal to merely spank anyone.
And those "Buffy the Robot" episodes - I always thought that the robot's bottom was perfect for spanking.
In later seasons, when Buffy and Willow were university age, that was when Dawn was the most spankable on the show. Her petty thefts and other antics could have easily earned her a warm bottom. I could easily picture either (or both) Joyce or Buffy spanking her. Or Willow, or Giles spanking Dawn.