Pretty good choices. When I think of this topic, I think of teams that either never won the Cup or never recovered from the loss, despite having won prior Cups.
1. 1979 Bruins. First thing I thought of when reading the topic is this series. Just look at the goalie's reaction when Lafleur's shot goes in. Totally crushed emotionally. Boston doesn't win another Cup until 2011. Their best chance to finally beat Montreal and likely win a Cup against an upstart Ranger team. Yet, they blow it.
2. 1993 Maple Leafs. This one seems easy too. They were 26 years removed from their last Finals trip. That's an entire generation of fans that never saw their team have success. Since 1993, it's been 27 more seasons (hard to believe) and still no team success. The fact that Montreal was waiting in the Finals that year was bittersweet. Fans were cheated out of a series that would have received tremendous hype.
3. 2011 Canucks. Especially with the riot. Especially since they also lost game 7 in 1994. With another riot to boot. It made the entire franchise and city look bad.
4. 1999 Sabres. Four Superbowl losses in a row in a different sport makes this even more painful. Here's a franchise and city that never won anything. I know they were underdogs, but the controversial way it ended still haunts their fanbase today.
I think these 4 absolutely belong. I would also indeed concur that the 1971 Finals between the Habs and Blackhawks as well. What all of these have in common were daggers twisting in the hearts of the losing team's fanbase. Not just how they lost, blowing leads while victory was seemingly at hand, but because all of these franchises had to wait decades until they got their shot again at winning again, or still are waiting.
There's less sting in the OP's post for an Oilers team losing, a Penguins, or Kings team because simply they had a few seasons around those collapses, where everything did go right.
Incidentally, the worst sting for Sabres fans was a 2001 loss in the 2nd round against the Penguins, with Darius Kasparitis's Game 7 OT winning goal.
Those era's teams, as good as they were at making the playoffs and getting deep, never, ever opened up their series at home. It always seemed to be on the road. This was a rare series in where they had home ice advantage--and it seemed nerves and all cost them both opening games--close ones, but losses. They then go to Pittsburgh and win by 3 goals each game, and come home to win Game 5.
Game 6 in Pittsburgh, they had a win looming, leading by 1 going into the final two minutes of the game, but a puck plops high up in the air, and lands right at the stick of Lemieux who ties it and they promptly lose in OT.
In Game 7, the team also had a lead in the 3rd, and give up both the tying goal and the winning one in OT (to a Kasparitis who picked up the puck during a jam in OT and threw it in front of Don Koharski (who said after the game "I'm not calling that"), minutes before).
That 2001 team was better than the 99' team that made the Finals.
Hasek left the team after that series, perhaps the team wasn't going to give him the tools he would need to succeed. The team did not sign holdout Michael Peca, who in many minds, would have been the difference for the Sabres that playoff year, over a difference of something like a million dollars (nothing to an owner who was later jailed for cooking the books). I think with him on the roster, they get past the Pens, and the Devils in the ECF (they were 4-0 vs. them in the regular season) and who knows after that.
Since that playoff loss, and Hasek leaving that season, the franchise is still trying to find its identity, having made the playoffs just 4 times in the 19 years, he has left.
