The Tiny Tim Scene from the Ghost of Christmas Yet To Come. The Ghost of Christmas Yet To Come (who reveals himself to be Pete) takes Scrooge to the future where Tiny Tim, Bob Cratchit's youngest son dies. The scene where Mickey as Bob places Tim's crutch by his tombstone with tears in his eyes is beyond heart-wrenching. It's a difficult scene already, but damn it, Mickey should not look like that. A tearful Mickey is very hard to watch.
Yet another reason this is one of a few, if not the only, incarnation where Mickey and Minnie have kids. Stomach that for a moment.
The Disney story book version features Scrooge's reaction to his own fate, which beautifully shows the real lesson about the future ghost. Scrooge shouts "No! I didn't want my life to end this way!" The "this way" part is a small but effective method of showing the true reason why Scrooge was afraid of this future. He wasn't afraid of dying, it was the way he dies that scares him; as a bitter, greedy, hateful old man who nobody loved and nobody mourned.
In a meta example, this was the very last time Clarence Nash provided the voice of Donald Duck prior to his death in 1985. You can practically hear the age in his voice!