What is the most significant moment in Stave Two of A Christmas Carol by Charles Dickens?

In Stave Two Scrooge is visited by the first of the three spirits Marley's Ghost told him would come. The Spirit of Christmas Past takes Scrooge to visit several scenes of his own past and one of his former fiance's past. Many of these events are significant, and we can see Scrooge already beginning to change and soften. The first indication he is softening is when Scrooge looks upon himself as a lonely boy at boarding school. All the other boys have gone home for the holiday, but he is left reading all by himself in the barren, cheerless room. This causes Scrooge to wish he had treated the Christmas caroler who came to his shop the previous evening with more kindness.

Next we see another Christmas where Scrooge is again left alone as a child, but his sister, Fan, comes to invite him home. Although not stated explicitly, we suspect this makes Scrooge think more fondly of his nephew, Fan's only son, since he grows uneasy when the Spirit points out Fan has died, leaving a child.

The next scene is the Christmas party of Fezziwig, Scrooge's former employer. The raucous and joyous event hosted by the munificent Fezziwig stands in sharp contrast to Scrooge's cold and cheerless counting house and his treatment of Bob Cratchit. This event leads him to wish he could speak to Cratchit, presumably to correct some of the unkind things he has said and done toward him.

Next are the scenes of Scrooge's break-up with Belle and her married life after she broke off their engagement. These are the hardest scenes for Scrooge to watch, and he ends up telling the Spirit to torment him no more and, taking the Spirit's hat, tries to snuff him out.

One could argue that the scenes where Scrooge begins to change and have regrets are the most significant, and the one where he realizes he has not been a good employer to Cratchit would seem to be the most important of those because a change in that part of his life would make the most difference since he works almost every day. One could also argue that the scene where he tries to snuff out the Spirit is the most important, for it shows us that Scrooge is not yet tender enough and that the coming Spirits have much more work to do before he truly is able to "keep Christmas in his heart."