FredBoat, largest open source discord bot.
Making all the things work + making it scale when demand kept climbing was a challenge where we had to learn simple stuff like postgres, working with 3rd party apis, generally good coding patterns and maintainable code, but also rather advanced stuff like making the garbage collector play nice, profiling memory leaks and optimizing the hot path, as well as high level topics like cutting the codebase into scalable domains and services.