worst collapses (updated with the 2007 mets), courtesy of baseball prospectus.
1. 1995 california angels
2. 2007 new york mets
3. 1951 brooklyn dodgers
4. 1993 san francisco giants
5. 2003 seattle mariners
6. 1969 chicago cubs
7. 1942 brooklyn dodgers
8. 1978 boston red sox
9. 2005 cleveland indians
10. 1983 atlanta braves
11. 1964 philadelphia phillies
12. 2002 boston red sox
13. 1908 new york giants
14. 1921 pittsburgh pirates
to give a flavor of how this is calculated, an excerpt (if you want to read the whole thing, i guess you'll just have to subscribe):
1. 1995 California Angels
Peak Playoff Probability: 99.988% after games of August 20th
Odds of Collapse at Peak: 8,332-to-1 against
Record at Peak: 66-41, 9.5 games ahead of the Rangers and 12.5 Games ahead of the Mariners in the AL West; 12.0 games ahead of the Yankees for the Wild Card
Record after Peak: 12-26
I posed this in the form of a trivia question to our internal mailing list: Which team had the worst playoff chase collapse of all-time? I got one guess for the 1964 Phillies, one for the 1978 Red Sox, one for the 1962 Dodgers (who rank 17th), and three for the 1951 Dodgers. Only Rany suggested the 1995 Angels, and that is because he has the best memory of anyone in our group (Christina Kahrl being a close second) and recalled Clay’s original article.
But in fact the answer isn’t even close. The 1951 Dodgers' odds were 384-to-1 against missing the playoffs, while the Angels were more than 8000-to-1 against. In fact, even if you ignore the Wild Card possibility, and looked only at the divisional race, this would still rank as the worst collapse of all time. With 38 games left to play, the Angels a combined 24.5 games ahead of the Mariners and Yankees, the two teams that would eventually pass them in making the playoffs. It took a perfect storm of events to knock the Halos out of the race—two distinct losing streaks of nine games apiece, plus the Yankees and Mariners each winning two-thirds of their games—and even then they still had the chance to redeem themselves in a one-game playoff against the Mariners. It's interesting to also note that if the Mariners had blown that playoff game, because the Angels played so badly that the M’s opened up a three-game lead at one point, the '95 Mariners would rank 13th on this list.
http://www.baseballprospectus.com/article.php?articleid=6764