I would have to say there are quite a few, but this may differ depending on whom you ask. I'll try to mention a few of the most important, in my opinion.
Perhaps the event that has the most impact on his life is when he meets Clarisse-she sort of "opens his eyes" to his meaningless life and pathetic society, even going so far as to ask if he's truly happy.
Another significant event is when he witnesses the lady choosing to burn herself along with her books. Montag really begins to question what would drive a person to go to that extreme-to die rather than have her books destroyed.
He also must face the task of burning down his own home after Mildred, his own wife, turns him in. He burns down his house, watches Mildred run out on him, then torches Beatty and the hound! Powerful scene. Now he's on the run.
Soon he attains his freedom and meets Granger. Together they watch a program that shows some random man who the announcer claims to be Montag being hunted and killed by the hound, and Montag cannot believe what he's seeing.
Finally, again in my opinion, a pivotal scene is at the end of the novel where Montag and his new friends witness their city being blown up, and a reference to a phoenix is made-another city will be reborn from the ashes.
There arguably could be more, such as the poetry reading with Mildred's friends, Mildred's overdose attempt, the meeting with Faber, and others.
The answer posted by sboeman is pretty good, but I believe that the pivotal scene in this book is when Montag watches the old lady destroy her home. During this point in the book Montag asked about the origins of the firemen and if they once extinguished flames rather than stoke them, the other fireman replied that the first fireman was Benjamin Franklin. To strengthen their statements they pulled out their 'rule book' which held the history of the firemen. As soon as they finished showing him the rule book the siren sounded (which meant that there was a house to be burnt down). Montag accompanies the other firemen to burn the house, when they locate the house that harbors the books they pumped kerosene onto the house and are about to light fire to it but the old lady accused of harboring the books pulled out a match and committed suicide by lighting herself (along with all her possessions) on fire. For Montag, seeing the suicide of an old lady made him curious about how a book could ignite such a passion in a person. He was curious about how such a small thing could be worth dying for.