Firebase Auth: Excellent for mobile and web applications. Relatively limited but easy to use. Integrates very well with other Firebase services.
Auth0: Comes with a rich and feature-rich user interface. Costs may escalate quickly, but it’s a safe choice.
Keycloak: A feature-rich enterprise service, offering a fully customizable user interface. There is no cloud solution, but it can be easily deployed on AWS using this CDK script.