Usually the one(s) I'm working on right at the moment.

But if I were to look back and decide which ones I still remember enjoying, a few come to mind.

For my day job, I've written two different (more or less) ORMs in my career. (For two different companies and in two different languages.) Pretty interesting stuff, that. The kind of project that you can sink your teeth into.

Neither one was a "I think I'll write an ORM" kind of situation. They were more like "oh, given these requirements, I think probably I'll need something that will work like X, Y, Z... oh, that's an ORM. Ok then, let's do this."

Side-projects-wise, most of the side projects I'd talk about are here. (Yes, AntiMS is me even though different username.) "hydrogen_proxy" got way less fun when I got stuck for like a year on how to make HTTPS work. But aside from that it was a lot of fun to work on. "codecomic" is quite interesting to work on as well.

And the project I've been working on most recently is a very configurable framework written in a combination of Go and OpenSCAD for creating bespoke, 3d-printable mechanical keyboards including keycaps.

Here's a preview of kindof roughly where I am with that mechanical keyboard project at the moment: