ASK-IH: What oAUTH sites do you use that users are comfortable/familiar with?

If I were going to start Plunker again, I would probably go with Auth0. I'm not adding value through a homebrewed login flow and increasing risk doing it myself.

Of course, I do work for Auth0 so I'll caveat this recommendation. But objectively speaking, I believe I would still recommend Auth0 even if I had no relationship with the company. Especially if I was planning on moving into a B2B use-case where single-sign-on might be a major value add.

Google and Twitter are pretty easy to get working. You should absolutely use an existing library to handle the OAuth flow.

I've been planning on adding Sign in with Apple as well. Could be useful especially if you have a lot of mobile users.

The typical trio is twitter, facebook, google in general case. If your product provide any benefit to the users by connecting these sites, for example integration to google services, sharing content to twitter etc, than users prefer to user these logins and implementing them is necessary anyway and many times you can even ignore email/pass auth. The same logic goes to developer tools or developer focused sites where the github, gitlab, bitbucket trio is the most common auth alternatives. If however, you are building paid SaaS users and developers still prefer email/pass as a primary auth method even if they ask social connection later in the app. If you plan to implement any of these alternatives to email/password, I would recommend using services like firebase auth, auth0 or amazon cognito to simplify the implementation and eliminate some security mistakes you easily can make if you roll out your own solution.