My favorite identity provider of all time is https://auth0.com
I was very impressed by their rules concept [1] that allowed me to create quite non-trivial authentication scheme in one day from scratch [2] They also pay attention to development workflows by having great logs and debugging tool chain as a part of their service, very cool!
[1] https://auth0.com/docs/rules [2] https://gravitational.com/blog/aws-github-sso/