That is not a simple question. I do not think that I can provide you with a single event of greatest importance. I will provide you with three events that I think are pivotal to the story, and you can pick one of them.
Event 1. Jonas being chosen as the new Receiver of Memory. I suppose this one would be the single most important event in the story, because it sets the rest of the story in motion. If Jonas were not the new Receiver, he wouldn't have done what he did later in the book.
Event 2. Jonas learning about pain and love. Those two emotions are so polarizing that they highlight each other. Because Jonas knows pain, he now knows what real love is, and he knows the Sameness is not real emotion. This is the moment when Jonas really starts to question the Sameness and his society.
Event 3. Escaping the community with Gabe. The reader doesn't know if Gabe and Jonas succeed in their escape attempt, but the act of escaping is important nonetheless.