7. 'Becoming' (Season 2, Episodes 21-22)
The second-season finale not only brings the entire series thus far to a head – it completely sets the tone for the remaining five seasons. This two-episode send-off tell Angel's entire history, from 18th-century drunk Irish cad to notoriously sadistic vampire to tortured creature of the night. “Becoming" also forces Buffy to fight her now-soulless boyfriend once and for all, while Willow begins to flex her witchcraft powers. The message: Going forward, the show would not be a simple, monster-of-the-week tale of a slayer balancing school and vampire-fighting. It would weave a TV mythology for the ages.