Oh no, I can't just choose one!
1. "The Gift". You knew that Buffy would be coming back, and yet, you still mourned the death with her friends.
2. "The Body", or would that be "Forever"? I'm not sure, but I lost my dad when I was a baby so the the end of the storyline where Dawn conjured her mother back and Buffy and her were fighting and then their [dead] mother knocked on the door with Buffy saying, happily-sort-of "Mom!" and then Dawn broke the spell. Anyway, it ended with the Summers sisters crying into each other's arms which kind of hit a sore spot with me.
3. "This Year's Girl" or "Who Are You?". Reasons stated above, I really felt for Faith when she found out her 'father' was dead.
4. S6 finale [Can't think of the name]. I absolutely hated the fact that Willow and Buffy were battling against each other. I really hate when friends fight, so I was pretty much beside myself the whole time. I was really devestated.