There is a strong case to say that given that the Leafs were 1 win away from the Stanley Cup Final.
However some Leafs fans might say it's 2013 against Boston because of how it happened.
Personally I'm still never going to get over the Leafs losing to Carolina in the 2002 Eastern Conference Final.
While the dramatics of 2013 is up there, I honestly felt worse about this years Game 7 loss, than that Game 7. In 2013, it was pretty clear that Boston was the better team, and the Leafs hardly deserved to be in the playoffs, let alone going to round 2. This year, with a lead going into period 3, it was a chance for this team to continue to progress forward. While this year doesn't put the Leafs back, it was still more disappointing to me than 2013.
2002 against Carolina doesn't get enough attention. Game 6, Sundin ties it in the dying moments of the third, all the momentum on home ice, and after failing to win the game after tying the game in the dying moments in Game 2, a chance at redemption Then Martin Gelinas....
2004 vs the Philadelphia Flyers was brutal as well. Still remember JR's stupid dance after scoring the OT winner. With Belfour in net, McCabe, Kaberle, Leetch on defense, Sundin, Mogilny, Roberts, Tucker, Niewendyk up front, that team is arguably the team that had the best chance at a cup since 1993, if not since that 67 team. In terms of heartbreak, that tops the list for me personally.
I'd also rank the 1999 Eastern Conference Finals series loss to the 7th seeded Buffalo Sabres ahead of the 2013 series as well. Winning Game 2 in a dominant way heading back to Buffalo, should have had all the momentum, and then bam! 3 straight losses by at least 2 goals, and we're done. That series was a brutal way to end things after that thrilling series against the Penguins.