Gomez
Drouin
Nothing really comes close to being as bad as those.
The Shaw trade was unnecessary, but the value is pretty much what those type of players go for (we traded Eller for a similar package).
The Weber/Subban thing was not catastrophic, but it just sent the team into a tail spin of aimlessness and mediocrity. Outside the COVID cup and the play in, the Weber saga had terrible teams. Aged the core without supporting it. If not for a fluke run, Weber's most memorable moments with the habs would have been breaking Gallagher's hand twice. Can't think of any other clear Weber memories during his time with the habs.
In retrospect, the Drouin trade is just brutal. Sergachev just turned 25. ( I will admit that my initial reaction was that it was a good trade).
The Kotkaniemi situations has the potential to be up there. It wasn't a trade per se, but there is still headroom for this to be completely awful.