Joss Whedon lists his 10 favorite episodes.
Not a one I disagree with, though I might rank them differently. His reasons ‘why’ just kill me.
7. Becoming, Part II (May 19, 1998)
The second-season finale, as Buffy rushes to stop the soul-free Angel from destroying the world. Willow does restore Angel’s soul, but not before he opens a vortex that will suck the world into hell. The only way for Buffy to close the vortex is to kill the man she loves — which she does.
Why does he like it? “Buffy loses everything. Also, it had a sword fight. I love sword fights.”