Cohen
Hellhound
Most of season 1 and 2 were filled with very dark episodes. It got super dark super fast. And then the Pylea arc happened, I think to lighten the mood. Seasons 3 and 5 were comparatively light compared to the first 2. Season 4 was complete garbage, but it was also dark. The darkest Buffy got was late season 2 with the Angelus arc, and season 6 with the addiction/depression storylines. But Buffy pretty much always remained light.