I used to think it was the special editions, but in light of TFA and Rogue One I might say it was maybe selling the franchise to Disney.
You were much closer with the first answer there, but to be more specific his worst decision was not making the special editions, but rather using it to bury the original editions. Nothing else he’s done has even come close, not even the PT.
There are not words to express the depths to which I agree with this statement. Well put, sir.