I found the following FreeCell games to be very challenging. A few I haven't solved yet, some took multiple attemps to win, and the rest are games that required much more than average effort. I play pretty well; consistently around 82% wins, in 1000+ game samples. I found these nineteen games to be noteworthy:
16731, 30596, 31729, 31052, 11522, 10574 (maybe), 17752, 27278, 30723,
22114, 11430, 21634, 25830, 9401, 28107, 6580, 23646, 29464, and 29435.
16731, 30596, 31729, 31052, 11522, 10574 (maybe), 17752, 27278, 30723,
22114, 11430, 21634, 25830, 9401, 28107, 6580, 23646, 29464, and 29435.

