Zack's death - makes me cry EVERY time. It might've been the bit afterwards, with all the flashbacks which make you remember the whole game and the music. Or, it might just be that I found Zack a much more likeable character, therefore I felt more emotional when he was killed despite knowing it would happen. Aerith's was kind of..lame, and Sephiroth makes me laugh afterwards haha.
Plus we actually WATCHED Zack dying, all bloody and messed up. Aerith didn't even bleed! So, maybe graphics all had a part in it too.
..Oh who knows T_T.