I went with Billy because the entire reason he acts like an abusive jerk is because of his father (who should’ve been turned into one of the victims of the Mind Flayer in Season 3, but what do I know?).
At least Billy redeemed himself to save Eleven and even apologized to Max for how he treated her. Not to mention he resisted the urge from the Mind Flayer to turn Mike’s mother into one of the Flayed.
Jason was just beyond reasoning and I’m still annoyed that he got Max temporarily killed. He was so shortsighted that it was amazing he lasted until the end of Season 4.