Hello Pelin, prepositional preference is normally about context. In this context both are acceptable. The first is in context to you personally... This is one of the most important moments 'that I've had' in my life. The second is from the perspective of live. Here you can understand the subtle difference and feel into the correct context to express what you want to communicate.

Both are used. "In" is more common. "In" would be more common when talking about a specific event in a specific life so far, while "of" would be more appropriate when talking about generalities that occur in everyone's life. My marriage was one of the most important moments in my life. Marriages are often among the most important moments of people's lives.