8. Mickey McFinnigan (Season 5, Episode 10)
When Peter Griffin accidentally killed Francis in the episode "Peter's Two Dads," the last words he heard from his disapproving father were, "You’re a fat, stinking drunk!"
Far from what he hoped his father would have said, Peter broke down while giving up his drinking habit for a crack addiction.
Brian ended up seeking a hypnotherapist to help his best friend recover. During a visit, Peter remembered Francis stating he wasn't his actual father. Once Thelma 'Mama' Griffin confirmed Mickey McFinnigan was his biological father and not Francis, Peter set out to find the dad he never knew he had.
Unlike Peter's previous alleged daddy, however, McFinnigan carried many of the same physical and character traits as his son, down to the obesity and dangerous relationship with alcohol. Even though there was a dispute about their kinship at first, the two instantly bonded as dum-dums.
It was a real shame McFinnigan dropped off from the series without having at least one more prominent role, as it explained a lot of where Peter got his... Peter-ness from and made for great drunken fodder. Plus, it's always nice to see a father and son getting along.