His worst decision was not completely breaking Anakin Skywalker.
He should have killed Padme earlier in the third film. He should have kept the children a secret from Anakin. He should have had Padme turn to Obi Wan for help, thus leading Anakin to suspect Padme and Obi Wan were having an affair. Anakin should have been the one following Obi Wan, leading to the duel happening earlier. Anakin in his rage still believes he kills Padme, Obi Wan still cuts Anakin to pieces. Emperor still builds Vader.
Anakin becomes Vader, Obi Wan is off caring for Padme/hiding the twins. She still dies in child birth from the injuries (not broken heart) while Darth Vader returns to Coruscant broken and filed with rage. Under the Emperor’s direction, he begins to wipe out the Jedi.
He had a lot of the right pieces for the PT, he just flubbed the order to put them in. He boxed himself in by wanting the story to be about Anakin, not Vader.