I would say the Isaiah Thomas trade.
As much as Stevens has been big, and the #1 pick is nice - this team hasn't had a 29 PPG scorer since the Larry Bird days, and we got this kid for [dang] near nothing.
Without Thomas, we don't make the playoffs in 14-15, probably don't make it in 15-16, and don't get past the first round this year.
Without Thomas, Horford likely never signs here, and we likely enter this free agency with zero desirability.