Worst - Not firing Therrien (and Lefebvre) during the 2015-2016 season - or at worst - at the end of it.
This is by far the worst move in the past 20 years. This was coming off our best season in a while with Price talked of as the best in the NHL with his hart/lindsay/vezeina - and a fantastic start after 20 games where we then went from first to last place. I don't care if you "feel" it's the coaches fault or not - the team record is still his responsibility. Failure at such a level should have been addressed asap and he should have been fired. The habs have never recovered since then. They've had some "on again/off again" success in the season, but have never seemed dangerous since then. That's when the rot started.
Clearly - not firing Therrien is directly linked in my mind to PK Subban trade. If you fire Therrien, PK doesn't get traded. I like Weber fine - in fact i liked Weber at the time of trade more than most of you today who have come around to liking him. That's not the problem, never was. PK Subban was the heart and soul of the team, and also of the fanbase. When he left, it was a huge deflating effect on fan interest, and the team hasn't looked the same since.
Obviously I realize that in the 2019 season, Weber is playing great and PK is having a horrible year. It's easy to say in retrospect it was a "good trade". I still don't buy it, worst trade ever. I'd like to think that if PK had stayed here many other things would have turned out differently too - both from team success perspective and also on an individual level for Subban, maybe his play doesn't fall off as much (still expect him to bounce back fyi).
Best - Not sure, maybe i'm forgetting some - but i do like one that was mentioned on the first page. Keeping Price and trading Halak. Especially considering it happened right after the playoff run, it was super gutsy. But in the long run this was the clear right choice (not for Price - he'd probably be a multiple time cup winner on almost any other team by now) for the Habs.
There were some other moves I really liked this decade. I loved the Vanek trade (even though longterm, it didn't lead to much). I loved the Radulov signing. I love that we made an offer sheet this summer (even though I fully realize in the end it means nothing). The Pacioretty and even Galchenyuk trades have turned out pretty good too.