I voted for Fredo. His betrayal and Michael's subsequent killing of him was probably the most emotional betrayal in all of the GF movies.
"Fredo, you're my older brother, and I love you. But don't ever take sides with anyone against the family again. Ever."
"Fredo, you're nothing to me now; not a brother, not a friend. I don't want to know you or what you do."
"Only don't tell me you're innocent. Because it insults my intelligence."
| 
