No.2 - Game 7 1978 NHL Quarterfinals vs Toronto
There will be those that believe this next loss was a painful lesson that helped the Islanders in the long run, although in the moment it served as the first real gut punch in franchise history.
The Islanders of the early 1980s were a dynasty, one of the greatest teams in the history of professional sports. The team in the late 70s was pretty dominant too, but their core of future Hall-of-Famers couldn't turn regular-season success and individual accolades into playoff success during the decade. The Islanders won the Patrick Division for the first time in 1977-78 with a 48-17-15 record and were considerable favorites against the Toronto Maple Leafs in the Quarterfinals.
The Leafs had beaten the Los Angeles Kings in the preliminary round, while the Isles had earned a bye and were waiting for them on Long Island. The home team won each of the first six games of the series setting up a decisive Game 7 at the Nassau Coliseum on April 29, 1978.
Denis Potvin scored an unassisted goal for a 1-0 lead 5:18 into the game, and things were going as planned. The Maple Leafs cooled the Isles' momentum and tied the game early in the second period. The game remained tied at 1-1 and headed into overtime, where Lanny McDonald beat Chico Resch at 4:13 to send the Isles home early. For many fans of the 1972 expansion franchise, it might have been the first time their hockey team made them cry.
The Islanders were even better the following season, registering a league-best 116 points. However, they were thwarted yet again, this time by the goaltender John Davidson and the upstart New York Rangers, losing 2-1 in Game 6 to lose the series. While losing one round short of the Stanley Cup Final and a chance at the Montreal Canadiens was bitter, there was no guarantee the Isles would win Game 7 if they won Game 6, which is why the 1978 game vs. Toronto is our pick here.
Who knows, maybe the dynasty would've started a year or two early?