I think it's gotta be the side-by-side branch editor that I used to write the entire script of Ladykiller in a Bind—it was kinda painful to actually work on, but it's also what enabled the whole game's development by giving me a new perspective on the way everything flowed! I don't think it would have been possible for the game to have worked out if I hadn't realized it was a problem that needed to be addressed.
A close contender is all the UI animations in GITCL, which were much more straightforward, but also a lot of fun to work on. I did a lot of frame-by-framing of some of my favourite animations to learn how smears work and I'm pretty proud of the results!
