The Buffy S8 comics storyline made me start wondering about seasonal Big Bads, specifically that of BtVS S5. I also started pondering how the Big Bad is never quite what we think it is.
My best example for that would be BtVS Season 6. It's been stated at various times that "Life" was the real villain of that year, and that the Troika and Buffy's arc represented the confusion and aimlessness that often characterizes one's early 20s. But it seems to me from the storylines that the real message was "we are our own worst enemy." Xander, Willow, and Buffy's storylines, and even secondary characters such as the Troika, Spike, Giles, Tara's etc. all seemed to come down to bringing ruin upon themselves by not taking responsibility for something. Had each taken the harder road rather than the easier one, much of the grief could have been avoided. The only person who seems exempt from this is Dawn, perhaps because she is the only non-adult at that point. I was struck by the fact that she never actually seems sorry for her actions in "Older and Far Away." Everyone else ends up the season plenty sorry though.
On the face of it, S1 seemed the clearest with the Master appearing in episode 1