What contributed most to Guy's transformation and why? Why does he change?

Arguably, it is Montag's meeting and subsequent friendship with Clarisse McClellan which contributes the most to his transformation. First, notice how Bradbury highlights the significance of their meeting through his description. In the few nights before meeting her, for example, Montag has some "uncertain feelings about the sidewalk," and the air seems to be "charged," as though somebody is waiting to meet him. Montag also detects a hint of perfume, another sign that somebody is about to make an entrance into his life. As a result, Clarisse's importance is foreshadowed before she makes her first appearance in the story, and she is the only character that Bradbury treats in this manner.
Secondly, when Montag finally meets her, notice the contrast between his physical description and hers. Montag, for instance, smells like kerosene and is wearing the dark uniform of the firemen. In contrast, her complexion is "milk-white," and she is wearing a white dress.  Bradbury is, therefore, highlighting the idea that opposites attract: although these two people are from completely different backgrounds, they are drawn to one another. Again, this emphasizes the importance of their meeting.
As the pair develop their relationship, her contribution to Guy's transformation becomes clear. Her topics of conversation make Montag think about the past. When she talks about not being afraid of firemen, for example, Montag remembers a moment from his childhood when his mother had lit a candle during a power failure. Montag links Clarisse with the "gently flattering" candlelight, implying that she has quickly become a source of comfort to him. Just like that candle, she represents a sense of hope when everything around him is dark.
Similarly, her questions prompt a deep sense of reflection in Montag. When she asks him if he is happy, for instance, Montag answers that such a notion is "nonsense," but, once she goes into her house, he begins to genuinely ponder this question. It quickly becomes apparent that he is not.
We see the full extent of Clarisse's impact on Montag when he finds out that she has been killed after being hit by a car. When he wakes the next morning, Montag has "chills and fever." This is the first time that Mildred ever remembers Montag being sick, a sign that he has been deeply moved by the news of Clarisse's death. Moreover, because he feels sicks, he decides to not go to work, prompting a visit from Beatty to reassure him of the firemen's importance in society.
Certainly, it can be argued that Faber has a strong influence on Montag's transformation because he becomes Montag's ally against the firemen. Together, they develop a plan to bring down the system. Similarly, the woman who is burned by the firemen in part one is also important in Montag's transformation. She makes Montag think deeply about censorship. Specifically, he thinks that if she is prepared to give her life in the fight against censorship, books might have some deeper, hidden value that is worth exploring.
It is Clarisse, however, who plays the most pivotal role in Montag's transformation. Long after her death, for instance, Montag still ponders the question of his own happiness. From the moment she asks him that question, he begins to evaluate several aspects of his life that he previously accepted as normal, like his marriage and his job satisfaction. Ultimately, Montag deduces that he is not happy, but, without this initial question, he would not have begun to look at books for an answer to his unhappiness. 
Clarisse, therefore, sets Montag on the path to transformation. Without their meetings and conversations, he would likely have continued working as a fireman, wondering occassionally about the books he burns, but never having a genuine interest to pursue their content further.