All three of them, really. I think Willow's reaction made me cry more than the loss of Tara. Same with the reaction to Joyce's death. Not that I didn't love those characters, the reactions were just able to provoke a bigger response than the action themselves. That's not unusual for me, though.
Jenny's actual death shook me to my core, her actual death made me cry, still makes me cry, and then it made me cry more seeing the reactions. I still don't know why Jenny really resonated with me so much as a character; She was there so briefly, but she really left an impression on me. I wish I could suss out why.