In A Christmas Carol, which character has the greatest impact on Scrooge, in your opinion?

It is a tie.  The two characters that have the greatest impact on Scrooge are the ghost of Christmas future as well as Tiny Tim. 

The goodness, holiness, purity, and youth of Tiny Tim contrast so greatly with Scrooge’s own despicable elderly character that Scrooge cannot but be moved by Tiny Tim’s death.  Everything Tiny Tim both says and does prove him to be a truly good person even down to his iconic statement:  “God bless us, every one.”  In contrast, everything that Scrooge does (in his past life) prove him to be a truly bad person.  When Tiny Tim dies (in the vision provided by the spirit) and Scrooge is allowed to live, the result is something not only unfair but also unjust.  This fact is a catalyst in Scrooge’s transformation.

Of course, Tiny Tim’s death can only be predicted through the other important character:  the ghost of Christmas future.  This ghost’s ability to reveal Scrooge’s name upon his own gravestone as the proof that Scrooge will eventually die and have to pay for his transgressions is the final element in Scrooge’s eventual transformation.