Auth0. I've tried a bunch of third parties and I found Auth0 to have good documentation, a great free plan, good support, and a user experience that is quite go

Auth0. I've tried a bunch of third parties and I found Auth0 to have good documentation, a great free plan, good support, and a user experience that is quite good. Some of the Rules/Actions terminology is a bit confusing, but overall the management UI and APIs are quite good. In addition, they have easy means of customizing the User object, querying external data sources, etc. Highly recommend.