Personally the coolest was the program I built for my fathers use on his job.

It was my first to be used commercially in the real.

That was a very big thing, I was 17 at the time an used turbo pascal 5.5 and he used it to compute how well all machinery was doing, they rented out diggers and other construction equipment to construction sites and manually compute this with a calculator took up to three days. (This was 1987 so there was not very many ready made programs for business, you often had to build your own)

With this program he had it done in around 30 minutes.

The next best was recently when I got my raft distributed consensus cluster server working. Its a little bit like zookeeper.
Building that purely from the research paper was rewarding but a bit of a challenge.