As previously mentioned in this thread, I think the deaths of Aeris and Zack are of a different nature and thus hard to compare. With Zack you go through the whole game following a young kid who grows up and faces hardship who you know is going to die, in vain, and in a bloody manner, which gnaws at you as you come to relate to Zack's character. One thing I thought was interesting was how FFVII was about creating your own person and coming to terms with it, defying the force that seeks to destroy your individual identity, whereas Crisis Core was about legacies and living on through others, creating an interesting push-pull dynamic. There was so little said about Zack in the original game, and although this is due to poor writing it actually makes CC more tragic, especially as you see Aeris, one of the few things left of his life, die. Anyway, I think Zack's death follows a more traditional narrative, building you up and then ending with the emotional release, whereas Aeris' death, as it is famous for, cuts right into the story, with little forewarning. When you look at it this way, her death is quite poignant and daring because of its sudden and completely frivolous nature, which makes it that much more tragic and realistic.