Personally I think, seasons 2, 3 and 6 are the best seasons. Best written, best acted, etc..
Season 6 is my favourite, for a couple of reasons. One of those reasons being its the first season I watched. Its the season that got me hooked, and I think I can pinpoint it to a single moment. Its the little moments in a thing that get me. Character arcs and plots and the intelligence of a thing are all good and well, but a simple image or a phrase hooks me. Buffy watching her friends at the end of season 2, and then walking away. Angel and Buffy staring at each other, and then him backing into the shadows, at the end of season 3. Season 6, for me just had more moments than any other season.
Bargaining part 1 and 2, I didn't think were particularly great. There was nothing in it that had me thinking wow. Afterlife had the first "moment". A very simple line from Spike.
SPIKE: But I want you to know I did save you. Not when it counted, of course, but ... after that. Every night after that. I'd see it all again ... do something different. Faster or more clever, you know? Dozens of times, lots of different ways ... (softly) Every night I save you.
When he said "Every night I save you", that got me completely hooked. His delivery, his expression.