Wow, this is tough.
I voted for The Body, for obvious reasons. The whole scene at Willow's gets me choked up every time.
Closely followed by The Gift, Grave (the end, oh boy), Becoming I & II, and I'm surprised no one voted for Hell's Bells. Didn't have me crying, but oh, it made my heart break for Anya.
I guess Buffy inspires a lot of tears, heh.