I doubt anyone here actually has enough knowledge of GMs throughout history to do a ranking.
Dorion didn’t do anything well besides drafting but I don’t know that he hemorrhaged our future either. Certainly didn’t help it. Maybe if he’d traded Stützle or Tkachuk, an actual major piece of the rebuild, it would’ve created irreparable damage. As it stands I don’t think he put us in a position where we can’t recover, he just made Staios’ job a lot harder than it needed to be.
It's more the death by a million cuts than simply 1 or 2 egregious things he did. Some bigger, some smaller. Like Mick said, he lasted a long time compared to others that would have been let go sooner, so the path is longer. The lists vary too.
Starting off by moving Zibanejad and finishing off with the worst UFA contract ever handed out in Ottawa (Korpisalo) certainly made him have a stink on entry and exit and is an ironic capture of the type of tenure he had.
Duchene trade for a ransom as the last ditch effort only months before deciding to rebuild. Obviously did not asses his team's state at the time well.
Only extracting long term value out of Karlsson from all the rebuild sell offs after the trade trees all collapsed is pretty glaring. Luckily, the Karlsson deal did still happen or it would have been an epic disaster.
Obviously Cat and Chychrun being expensive acquisitions not working out and not being long term pieces was big.
A few reaches at the draft like Boucher, Jarventie and some others I can't recall atm hurt them. Every team does it, but that regime had a couple that were perplexing based on where the guys they did take were anticipated to go and who else was there.
Mismanaging of the money when Melnyk did open up. The Dadonov & Murray signings being at the forefront of that.
Smaller things like saddling himself with a long term Zaitsev unnecessarily or not being able to hit with a competent tender, or at least during the time he had them.
Some of the short term stuff like moving Gus for Talbot or giving Colin White a big payday came back to bite him as well.
Paying to get rid of numerous of the self inflicted mistakes above cost them assets on top of it.
Dadonov fiasco.
I feel like his inability to effectively manage the PR for the team with a few train wreck moments, as basically the only guy they were rolling out there for a while, just sort of hammered it all home as a total package of looking bad as a manager and as an organization. Although nothing he said will ever top the Nick Rutkowski quote that Senators fans should be excited by the existence of the salary cap floor that the Senators were required to spend to. While they were like 8-10 mil below that number in real salary spending.
All in all they are basically starved for assets at this point even though they may be okay with competent management. I don't think that's reasonable given where they came from and burned through and I don't know who else you would hold accountable for it.