I spent 5 years as sole programmer on a Doom 3 total conversion. Joined the team as a shitty C programmer that didn't know C++, and just hit the books and got rid of the social life. The successful part is that we actually finished it.

It's just a game mod and my fist foray onto a new language, and current project will hopefully soon replace it as my biggest & best show of skill, but I'm still proud of it.