Not really a staking, but the most memorable dispatching of a single vampire on either series to me was when Buffy stabbed Angel and sent him through the portal into hell at the end of Season 2. Most memorable because of the drama, the presentation by the actors, and because of the effects that in had on the characters, primarily Buffy. Doing that to Angel was without a doubt the most difficult decision and the most emotionally scarring single thing that Buffy had ever done up to that moment, and the most shocking thing that Angel, up to that time, had ever done. Sarah and David played the moment superbly, with grief far beyond tears on Sarah's part and shocked betrayal on David's, and they didn't have to speak a single word to convey everything that they were feeling at that moment. It was truly one of the most heart breaking moments of the entire series, surpassed only slightly by the deaths of Joyce and, of course, Buffy herself.