If we are talking whose betrayal is the biggest on an emotional level and storywisw it is fredo, which you mention but second is a three way tie between Fabrizio, Paulie, and Carlo. Each one wounds the Corleno family in ways they never recover. One takes the don out of the picture and puts Michael on a path he didn’t want. One takes away the type of women Michael would need to be in the life he chose Kay because Kay knew the man he wanted to be not who he had to be and became for his family. Apelonia would of been the rock and emotional support he needed in godfather 2 and Carlo removed Sonny off the board and once Michael removed him off the board it caused a rift between him and Connie that doesn’t heal until their mother dies.