I feel a bit ashamed posting this, compared to some of the amazing things you guys have built.
Coolest thing I have built was my first app:
https://play.google.com/store/apps/...
Story:
It was back around new years 2014-2015. I bought a charango and started playing some gigs. I carried around a book with chords. I thought it was a bit annoying to have to take it with me. Looked for an app and there wasn't any (today there are 2-3 other). So I decided to make an app.
Bare in mind that I had just a bit of experience with C from university. No OOP. So I went on youtube and started watching some tutorials while I developed it. Learned by trying. Trial and error.
After around 2-3 months of working on it every day after class until going to sleep, it was ready.
I decided to put it on play store for other people to use. Turns out there was a need. I got 10,000 downloads in less than half a year (it is quite a niche, so unexpected). Since then it has stayed around 6000 installs on active devices.
It is my biggest personal project success.
Since then, I have continued making apps in my free time, getting better and more professional. But none has come even close to that ones popularity. My plan is that to mark the 5th anniversary, I am working on a v2.0 (complete rewrite) with new features and instruments.
Sorry about tl;dr