I think I've learnt something worthwhile from nearly every project I've been involved with. If I had to pick one however:

Started an open source project designed for projecting multimedia content during church services as procrastination from final year undergrad revision.

Fast forward nearly a decade, and I've learnt tremendous amounts as a result of starting it - dealing with everything from GStreamer on a native C layer, right through to WebRTC stuff (STUN, TURN, ICE, etc.) at the other end. What started as some odd attempts to show text and images on a screen in a user friendly fashion has grown tremendously, and is now used all over the world.