Out of thousands of scenes across 144 Buffy episodes, this is the only scene that no matter how many times I watch it, I must physically react in some way. I guess it is the adrenaline needing some way to leave my body, so I yell or clap or punch the air with my fist. (Yes, this is incredibly embarassing, but true.)
I remember seeing it for the first time & I was so wrapped up in evil Willow and the bitch fight to end all bitch fights that Giles was the furtherest thing from my mind. I hadn't given him so much as a fleeting thought, so when he showed up there in the doorway ... "I'd like to test that theory." PERFECT.