What decision did Tony make that had the biggest domino effect?

I think it was Tony’s hesitation to kill Tony B. It massively angered both Phil and his own crew. The blatant nepotism was on full display for everyone to see. It’s the closest he ever comes to facing a mutiny, and Phil never forgot how Tony stole the satisfaction of him getting to kill Blundetto. I believe this directly lead to his death in season 6.