I think every project I find an interest in pursuing is definitely the coolest for me but if I had to choose I think my coolest project was creating my own IDE for competitive programming. it was mostly inspired from sublime with a couple of functionalities purely for competitive programming. I have it all open source just wish i had more time to work on it!
in case you're interested you can check it out on my github (beware it has the shittiest documentation haha)
http:///www.github.com/...