I'm going to ask two questions. Just two questions.
It's a koan.
Meditating on these questions will reveal everything to you.
The answer to these questions is not the answer to which project to contribute to, but meditating on these questions will reveal the project to you when you encounter it.
Here are the questions:
1. How many commits does it take to become the #2 committer in the world on Immutable.js?
2. If you did that, where would you end up working?
About me: I spent three years as a recruiter analyzing GitHub to find developers. Also, I'm a professional software developer, and I've worked in open source for over a decade. Read about how I got my last job by committing to open source.