There are episodes of Buffy that literally make ball my eyes out everytime eventhough I've seen them like 30 times each.
The Gift is maybe the biggest cry, her speech to Dawn and everyone's reaction to seeing her body on the floor, especially Spike, its heartbreaking. The Body makes me cry pretty much the entire time, start to finish I'm a mess. The Prom and her getting the class protector award, but at least thats a happy, proud cry. In Passion when Giles calls Buffy's house and we just see Buffy's face drop and then Willow gets on and she breaks down, it just kills me. When Oz leaves after the whole Veruca mess in Wild at Heart, I completely share Willow's total heartbreak everytime, man that girl is a pro at tearing your heart out with her performance. Anya's walk of shame in Hell's Bell's, again it completely breaks my heart. Of course Tara's death in Seeing Red, probably mostly due to Willows reaction to it. And finally the subtle moment in Potential when Xander says to Dawn, "You're not special, you're extraordinary." Its just such a beatiful heart-warming moment.