What's the best way to rate programming skills?

In an interview I was asked to rate myself as a programmer on a scale from 0 to 10. I had a really hard time answering that question because I had never really considered it.

So the first part of my question is: what makes a programmer rate higher than other programmers? Is it proficiency in multiple languages? Is it the amount of Github repos you have? Or perhaps how fast, clean, and modular your code is?

This leads to the second part: how do you build a portfolio that showcases these skills? This is easy if you're a graphic designer: you simply have a page that has all your designs on it. But what does a good portfolio look like for a developer?