Honestly? It’s highly individualized.

The best one is probably one that you already use and care about, and want to improve.

Choosing some random project will not really provide motivation in learning the code base, figuring out how to implement a change, etc.

Agreed. Explaining the risks has always worked better for me than explaining the benefits. Most companies only care about their bottom line.

Yeah. Chromium has something like > 30 million lines of code. Browsers these days are like mini operating systems in a way.