I think the coolest project I did was a few years ago, it was actually a Minecraft plugin.

I decided to learn Java for Minecraft, and a few months after I started learning Java, I was approached by someone who'd like to work with me to create this full-blown Gun Game style gamemode for Minecraft. I made it clear I didn't have the most knowledge, but I was willing to learn.

We began working on the project, the projects main class was bigger than any project I had worked on. Within a few months, it became one of the more popular plugins out there, even though we were still in an alpha mode. Had nearly 1,000 servers running the plugin, over 10k+ players total testing out the plugin.

Cause of this project, I learnt how to properly organize my code, how to make it efficient, learnt how to network, learned how to properly secure and verify anything being sent by the client, working with dependencies, adding features that can support a bunch of other plugins that other developers had, and a bunch more.

Sadly we couldn't finish the plugin anymore, so we gave someone else the source code who has kept it updated to this day. (I know I didn't provide much insight into what I'm saying and just gave a general overview, got a killer headache.)