What ghost has the best effect on Scrooge?

No single ghost has a "best" effect on Scrooge.  In fact, the messages of all of the spirits, including Marley's ghost, are essential in helping Scrooge become a better person.  Looking at what Scrooge gains from each of them will help illuminate their joint effect.Marley's ghost not only shocks Scrooge, he also gives him a direct warning regarding his chains and the connection to the chains that Scrooge is forming in life.  By starting with Marley's ghost, Scrooge is given a touchstone and a level of familiarity with the ensuing events in his journey.  While the spirits do wish to shock him, their entire goal is to help him change.  As terrible as his afterlife is, Marley's  afterlife is significantly better than the afterlife that awaits Scrooge if he does not change.The Ghost of Christmas Past is essential to reminding Scrooge of where he came from and of the kindness that Scrooge experienced earlier in his life.  Through a glimpse of Scrooge's childhood at the boarding school, the spirit reminds Scrooge of what it is like to be lonely and abandoned. However, the spirit also shows Scrooge what it is like to feel love through Scrooge's sister.  This scene also reminds Scrooge to love Fred, the only family he has left.  Through Fezziwig, the spirit shows Scrooge how a boss should act.  Through Scrooge's breakup with Belle and the view into her life on the eve of Marley's death, the spirit shows Scrooge what he could have had.The Ghost of Christmas Present shows Scrooge a number of important scenes that help him to change.  By showing Scrooge Cratchit's meager home and Fred's dinner party, the spirit shows Scrooge the kindness, love, and forgiveness of the holiday season.  Though Scrooge has mistreated all of them, they all toast him and wish him the best.  Further, the spirit also introduces Scrooge to Ignorance and Want, the manifestation of humankind's worst traits.  Finally, the spirit also throws Scrooge's own words in his face to show him how calloused Scrooge has become.Finally, the Ghost of Christmas Yet to Come reveals the future that lies ahead if Scrooge fails to change.  Rather than hint at Scrooge's afterlife, as Marley did, the spirit shows Scrooge the world without him.  Tiny Tim dies, and Scrooge dies cold and alone; his possessions are stripped from him and pawned off.  The only good that comes from his death is the relief of those who owe him money.Each spirit is essential to Scrooge's movement from greedy to giving.  Without the combination of past, present, and future, Scrooge would not have been able to change.  This reflects Dickens's own belief in the importance of experience.  Experience is important;  it comprises who we are and who we will become.