I've been seeing this circulate around my flist on livejournal, and decided to go for it. For the month of April, I'll be answering questions related to my writing.
It's hard to decide, really. But overall, I'd say that the short story I had the most fun on was a fanfic for "Flint the Time Detective" that was a retelling of Cinderella (although it was really more like the characters putting it on as a play, just with no audience). That one even earned me fanart, though it was lost in a computer crash many years ago ;_;. But it was just very silly and fun to do.
Of the longer (and more recent) stories I've written, I enjoyed my 2008 NaNoWriMo novel for much the same reason, it was just a lot of silly, intentionally overdramatic stuff, and was technically a reimagining of my 2007 novel, which turned out to be not as much fun as I thought it would be, mainly because the main character didn't go anywhere, and most of the action happened offscreen as it were. So in the 2008 novel, the main characters explicitly had the power to travel to different worlds, and were thus obligated to go all over the place. So yeah, a lot more fun the previous year.