The 1999 National League Championship Series.
The Mets, Wild Card winners, faced the Atlanta Braves who, this being the 1990s and all, were the NL East winners. The Braves were 9-3 on the season against the Mets and many members of the Braves, led by huge asshole John Rocker, mocked them and their fans, especially when, two games back in the Wild Card with three to play, the Mets looked unlikely to make the playoffs. But they finished the season in a tie for the Wild Card with the Reds, and won the one-game playoff on my one true love (well, after Andy Pettitte) Al Leiter's complete game two-hit shutout. Both the Mets and the Braves won their division series in four games, leading to a showdown in the NLCS.
The Mets' starters gave their teams very good starts, but behind Maddux, Millwood's ridiculous career year, and Glavine, the Braves took a 3-0 series lead. At the time, no team had ever come back from that deficit or, in that situation, pushed a series to a Game 7, and only one team had even forced a Game 6. In Game 4, the Braves had a 2-1 lead going into the bottom of the eighth, but the Mets managed to score twice off of their torturer John Rocker, and Armando Benitez somehow held on for the save.
Game 5 was the longest postseason game ever, at nearly six goddamn hours, until games in 2004 and then 2005 beat it (one of the worst parts of 2004 was not only that we lost in such a frigging awful way, but that the games were so unbearably long AND THEN WE LOST THEM AAARGH). The Mets scored two runs off Greg Maddux in the first, but he settled down after that. The Braves countered with two runs of their own in the fourth. Then, no scoring happened until the top of the fifteenth, when Keith Lockhart tripled in Walt Weiss. All the Braves had to do was hold on, and they would go to the World Series. But they didn't hold on; a bases-loaded walk and the generally awesome Robin Ventura's "grand slam single" led to the Mets' win.
Game 6. Al Leiter, so awesome in the tiebreaker game, obviously didn't have it. In the first inning, he hit two batters and gave up five runs, and was done before he could record an out. The Mets were held scoreless by Kevin Millwood until the sixth, when they scored three runs to make it a 5-3 game. In the bottom of the sixth, however, the Braves responded by scoring two insurance runs. With a comfortable four-run lead in the seventh, playoff pitcher extraordinaire John Smoltz came in out of the bullpen to shut the Mets down and send the Braves to the World Series.
However, maybe not fully warmed up, he let the Mets score twice, then, ahead 7-5, faced the injured but still feared Mike Piazza with a man on base. Piazza hit his first extra-base hit of the postseason, a home run. Tie game. In the top of the eighth, the Mets scored once to put themselves ahead 8-7, but John Franco couldn't hold that lead and the Braves tied it up in the bottom of that inning. Armando Benitez and John Rocker delivered a scoreless ninth inning, as the game moved into extra innings.
In the top of the tenth, the Mets scored off of Rocker in his second inning of work (hmm maybe he's not Mariano Rivera), but like Franco, Benitez couldn't hold the lead (hmmm maybe he's not Mariano Rivera either), and the tenth inning ended in a 9-9 tie. Some guy named Russ Springer shut the Mets down 1-2-3 in the top of the eleventh.
Bottom of the eleventh. For the Mets, out comes Kenny Rogers. He allows a leadoff double to Gerald Williams, then Bret Boone bunts him over to third. Looking for the double play, Rogers intentionally walks the dangerous Chipper Jones and Brian Jordan. Up steps Andruw Jones (this is when he was good, before he was fat and sucked). And the Braves win the game, and the series... on a walk-off walk.
I think this is like the worst way to lose a series, especially considering that the Mets had to watch the World Series be Yankees vs. Braves. At the time, that was the equivalent of all the agony the Mets went through with the Yankees/Phillies World Series this year. It might be worse than the 2006 NLCS for Mets fans. I mean, they went to the WS the next year, so it had to hurt a lot less, but yeah. If this happened to the Yankees, I think I'd throw shit at the wall, but because it's the Mets, LOLMets.