While it doesn’t cover all of the use cases of a CLA, I believe the DCO is the best form of a CLA: Developer Certificate of Origin - Wikipedia
Of note, from what I understand, most large companies (who typically own the copyright to your work out of office hours), will require any project you might want to contribute to in personal time to have a CLA in-place.
I personally am against CLAs, that aren’t akin to the DCO.