What event most changed Scrooge during the visits of the three spirits?

Scrooge experiences a dramatic change of perspective, what one could even call an epiphany (which is an appropriate word given the Christmas setting), by the end of this story. The visits from the three spirits each bring significant realizations and changes, so it is difficult to choose one as the most important. But I think there are three moments which stand out for the deep impact they have upon Scrooge. First, the moment during the visit from the Ghost of Christmas Present when he sees Tiny Tim for the first time and feels a jolt of compassion.

The other two moments occur during the visit from the Ghost of Christmas Yet to Come. He sees a tombstone with Tiny Tim's name and his family grieving; Scrooge feels indirectly responsible for Tiny Tim's death, knowing that the Cratchit family's misfortune is linked to their poverty, which he is in a position to change.

He is then shown his own tombstone, and it is in this moment, catalyzed by fear of death, and regret for all his past misdeeds and all he has left undone, that he vows to change.

"Spirit!" he cried, tight clutching at its robe, "hear me! I am not the man I was. I will not be the man I must have been but for this intercourse. Why show me this, if I am past all hope!" 

For the first time the hand appeared to shake. 

"Good Spirit," he pursued, as down upon the ground he fell before it: "Your nature intercedes for me, and pities me. Assure me that I yet may change these shadows you have shown me, by an altered life!" 

   The kind hand trembled. 

"I will honour Christmas in my heart, and try to keep it all the year. I will live in the Past, the Present, and the Future. The Spirits of all Three shall strive within me. I will not shut out the lessons that they teach."

After this, Scrooge awakens from what may have been a dream (the novel does not clarify if these visitations were real or a dream, but it hardly matters since the impact on Scrooge is suitably powerful) and is utterly changed. He decides to seize the opportunity to do good. he runs into the street shouting "Merry Christmas!" to everyone who passes by, and his greeting is returned warmly. He tells a passing boy to go and by the huge Christmas goose hanging in a shop and to have it delivered to the Cratchit house. He offers Cratchit a raise in salary, and also offers to help financially with Tiny Tim's illness.

We learn that Scrooge did indeed turn his life around, that Tiny Tim did not die, and that Scrooge continues to honor Christmas with joy and celebration. The opportunity to change his life is one that is inspired by the dream/vision of the spirits, but that ultimately comes from Scrooge himself.