The answer is "all of them!"
ESR wrote "Every good work of software starts by scratching a developer's personal itch." You'll be much more effective working on a program that you use and want to make better, compared to a beginner-friendly project that you have no interest in.
Instead of looking externally for projects, look inwards. Think about the software you already use, and how it could be better: that stupid annoying behavior, or that obvious improvement, or your idea for a cool feature. And then set your sights on that!
An example: I was a big fan of the game Angband, which I played on Mac OS 9. When OS X came out, I wanted to play Angband again. So I "carbonized" it, so I (and others) could play it on OS X. I had never carbonized a program before, and that's how I learned.
I apologize for not really answering the question, but I have known aspiring programmers give up because they got bored. "Not being bored" is really important, more important than "not being frustrated."