Dave Pettit did an excellent job of answering a question I posed on Retrolist (the discussion group for Retrosheet). Since the White Sox have done so poorly in the 2nd half this year after doing so great in the first half, I wondered where their collapse would rank. The Sox were .654 in the first half and since then they are .453, for a drop of .201.
Dave did a great job of compiling the 100 biggest drops and gains in percentage going from the first half to the 2nd half. Dave included ties in figuring the percentages and I took his win loss totals and used only them to calculate the percentages. I will try to adjust this when I get a chance (I have since done that). Mike's Baseball Rants has even more info on this (see link at the end).
Below are the worst 25 drops in pct.
The Sox this year are currently 22nd. It looks like they would be the only defending champions in the worst 25.
Below are the best 25. The Braves of 1914 are sometimes known as the "Miracle Braves." They won the pennant and world series (they were 34-43-1 after play ended on July 17 according to Retrosheet in last place or 8th place 11.5 games out-they finished 94-59-5 and were 10.5 games ahead of 2nd place NY-the Giants were in 1st on July 17 by 3.5 games)