Alright then.
Zack's demise and the inheritance of the Buster sword is the most memorable moment of the compilation for various reasons. To go into them all would take far longer than I'm sure everyone wishes to read, so I'll only go into a select few, the reasons that are true of both the original version, and the version presented in the end of Crisis Core.
The viewing of Zack's demise shows us a man, trying to get himself home to his girlfriend and to safety, but moreover, it showed us a man who was going to make sure his comatose friend made it to safety too. It showed us a man, within miles of his goal, who is nevertheless fighting, to the death, to save that friend even as it means he cannot return to the woman he loves. It showed us a man who died so that his friend could survive.
Nextly, it shows us death comes to everyone, and by any hand. While Aerith's demise was memorable, it showed us that the final boss of the game could kill someone, what Zack's demise showed us was that death could come at any hand, even the hand of the MP grunts you fight at the very beginning of both FF7 and Crisis Core. It made you appreciate the exploits of the hero more, because it highlighted their fragility, and made their deeds the more impressive because of them. It also added a further point to the suddenness and the sadness and brutality of death, both in FF7's recollection by Cloud in which the MPs finish him off once and for all with a riddling of machin gun fire, and in Crisis Core's version with Cloud's scream and Aerith's shock.
But lastly, it is memorable, because while it highlights the sadness and brutality of death, it also shows us that death is not truly the end. In FF7, this was Cloud holding the Buster sword and telling Zack he would live on for him. In Crisis Core, this was the hand grab with Angel, Zack's calling Cloud his living Legacy, and in both cases, Cloud managing to walk standing up only after taking up the Buster sword, and renewing Zack's struggle towards midgar. The passing of the Buster sword isn't just a handing of a weapon on, or a keeping of a memento, but a passing on of honor and dreams, just as with Angeal and Zack before, and as a continuation of Zack's role as a hero through Cloud. It is not the end of Zack's story as a hero, but as Cloud notes at the end of Advent Children Complete, it is the start of one. His.
Before Crisis Core, Zack's death and the passing of Buster was a scene that gave a little bit more information on who Cloud was trying to be, on why Cloud reminded Aerith of her old boyfriend, and on how Cloud came to be in Midgar's slums after Nibelheim.
Now, it is the defining point between the two major eras depicted in the Compilation, the capstone to the themes of life and death that prevail throughout the series, and as the end of Zack's life, but not yet the end of his story.
And Quex, what ARE you gasping at?