I think season 7. Atleast in the way the council and the shadowmen would have defined it. She was focused, ready to make tough decisions, able to lead reasonably well (still had some kinks to work out) and a experienced fighter(actually, when I think about it, slayers were suppose to be followers not leaders, so scratch that, the watchers wouldn't want that). I'm sad to say that I would have to agree with them. While I think Buffy was a better person in season 5, and the other seasons, she was a better slayer in season 7. Or a better warrior, since I want to include her leadership qualities.