Jacob Marley had the greatest effect on Scrooge, because he created the opportunity for his transformation.
There was not just one character who had an influence on Scrooge. However, since Jacob Marley is the one who brought the three ghosts to Scrooge, and set up the transformation, I suppose he would be the most influential person if you take the story at its face value. If you assume that the story is not "true," then I would suggest that the answer is Fred. It all depends on your point of view.
Let me explain. If the ghosts are real, then Jacob really did visit, and he really did send Scrooge three ghosts that took him on a journey that led to his transformation. If the ghosts are not real, and the story did not happen, then all of this was a dream, influenced by Fred's visit. Fred visited Scrooge and invited him to dinner. This piqued Scrooge's conscience, and caused him to dream the whole thing.
Let's proceed with the first theory, that the story is real. In this case, Jacob Marley, Scrooge's friend and former partner, is the instrument of Scrooge's transformation.
[Scrooge] then made bold to inquire what business brought him there.
“Your welfare!” said the Ghost.
Scrooge expressed himself much obliged, but could not help thinking that a night of unbroken rest would have been more conducive to that end. The Spirit must have heard him thinking, for it said immediately:
“Your reclamation, then. Take heed!” (Stave One)
Jacob Marley has been watching Scrooge for a long time and decides that he needs a lesson so he does not face the same fate that he did. He somehow arranges this intervention for Scrooge. Because of this, all of the visions that Scrooge sees are designed to help him realize that he is on the wrong path, and get him to re-evaluate his life. This makes Jacob Scrooge's greatest influence.
The ghosts know all about Scrooge, and use this to manipulate him. We see this specifically when Scrooge's words about the "surplus population" are thrown back in his face. Scrooge told the two men who collected for charity that if the poor were going to die, they should just go ahead and do it because they were not worth living. Later, Scrooge regrets these words.
“Spirit,” said Scrooge, with an interest he had never felt before, “tell me if Tiny Tim will live.”
“I see a vacant seat,” replied the Ghost, “in the poor chimney-corner, and a crutch without an owner, carefully preserved. If these shadows remain unaltered by the Future, the child will die.”
“No, no,” said Scrooge. “Oh, no, kind Spirit! say he will be spared.”
“If these shadows remain unaltered by the Future, none other of my race,” returned the Ghost, “will find him here. What then? If he be like to die, he had better do it, and decrease the surplus population.”
Scrooge hung his head to hear his own words quoted by the Spirit, and was overcome with penitence and grief. (Stave Three)
He also has to face his words about prisons and workhouses when confronted with Ignorance and Want. The Ghost of Christmas Present also confronts him with his own words. When Scrooge asks if the children have no "refuge," he is reminded of his cruel words, and becomes repentant.
Scrooge is clearly reforming by this time. He asked about Tiny Tim's fate, and now worries about the children. By the time he is confronted with the Ghost of Christmas Yet to Come and his headstone, Scrooge promises that he is a changed man.
“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!” (Stave Three)
Here Scrooge specifically says that it was the interventions of the ghosts that have changed him. This is before he is returned to the mortal world, when he realizes that he has seen a future where no one cared about him. When Scrooge saw this future, he was already a changed man in his heart. The future he saw was the future of a Scrooge he no longer was.
By the time Scrooge meets with Jacob Marley, he is already reflective. The visit from his nephew is what got him thinking. Fred invited him to dinner, and although he turned him down, this invitation had an effect on Scrooge. It softened Scrooge. Were the ghosts real? Was it all a dream, brought on by guilt at seeing Fred? Either way, Scrooge changed because of it, and changed for the better. These two men, Jacob Marley in the ghost world, and Fred in the real world, helped Scrooge see that he could be a better man.