I would love to know it. I use GH Actions to test my open source code on MacOS, Windows and Linux FOR FREE which is amazing. Without them I just wouldn't test on anything other than what I happen to have locally or what TravisCI/CircleCI had in their free tier before GH Actions made things so much easier.
That is what I meant by the BEST FREE CI provider. It is not the best if you have money for something better.