In A Christmas Carol, the Ghost of Christmas Past is the first to visit Scrooge. While Dickens does not specifically state that this ghost is the most important, he alludes to this fact in two important ways.
First of all, the physical description of the Ghost of Christmas Past emphasises its importance in the story. It springs a "bright clear jet of light" from its head, is dressed in the "purest white" and its belt "sparkled and glittered." This angelic description contrasts sharply with the other ghosts who visit Scrooge that night: the Ghost of Christmas Present is far more ordinary and human while the Ghost of Christmas Yet to Come is simply a hooded figure. No other ghost receives as much attention from Dickens nor such a wondrous and detailed description.
Secondly, it can be argued that the Ghost of Christmas Past shows Scrooge the most important powerful images in the book. These images, of his childhood and his former relationship with Belle, not only explain how Scrooge became so miserly and greedy, they also prompt the important process of transformation. Without seeing these shadows of the past, Scrooge would never accept that he needed to change and he could not have achieved this without the Ghost of Christmas Past.