I guess still this one: https://paulsgames.itch.io/ostarkdidi
Sometimes simple game mechanics just work out really well. I had no problems coming up with 70 different levels/puzzles for this game (well, it still took a lot of time), because of the depth of the game mechanics and all the different possibilities they give. Because of how well it does in terms of depth vs complexity (see e.g. Extra Credits), I'm really proud of this one.
I've also used this six year old game to practice new tools and skills over the years (drawing with Krita, adding online leaderboards, extending my custom engine to the web, writing dynamic music, etc.)
