For me, it falls between Zack's death (CC version), Sephiroth's advent in AC/ACC, the scene where everyone returns to the Highwind near the end of FFVII, and Cloud beating Sephiroth in Nibelheim -- original FFVII presentation of said scene, thank you very much.
The first and last of those in particular is hard to choose between, but I think I'm going to go with the revelation from Nibelheim. It solved the game's big mystery, and was just such an all around well-directed scene, despite how many limitations there were for the developers then.
So, I really think that Zack's Death should be combined in with his DMW decay, because they're really all part of the same scene imo, and it prevents the large possibility of it taking up 2 places in the finals.
I agree with Ryushikaze -- they're separate events, just like Sephiroth's resurrection in AC/ACC is separate from the final battle of AC/ACC, despite one leading directly into the other.
By the way, Mako Eyes, small nitpick: Cloud and the team snowboarded at the Great Glacier, not Gaia's Cliffs.