Look, we could probably go back and forth on this until I have to leave for work in the morning*g* Since we'll probably disagree on just about anything in this respect. Spike's story of redemption, starting in s2 with Becoming, when he chooses love over the apocalypse, to s4 with the chip, leading to his voluntarily fighting to regain his soul is probably the story that makes Buffy worth watching for me. It's the one story that keeps strong all through the series, even as most of the other actors lose interest in their characters and start getting boring, despite having much more screentime in s7 than Spike ever did.
Personally the thing that dragged down Buffy up to s6 was Willow, Joss' Mary Sue. (and don't get me started on how much I utterly and completely despise her and every facet of her character)And then in s7, Andrew and the Potentials.
Spike is what saved the show and kept it worth watching till the end.