To be honest, there's so much good open source code out there, that I feel a better question would be "If you could fully understand one cohesive software unit, what would it be?"
Too often I feel overwhelmed by everything I want to absorb given the limited ability of my brain to fully absorb and comprehend it all. Constantly battling down an army of open tabs, against myself who opens them faster then I can process and close them, lol.