Easiest and most secure (in the sense of not getting things wrong) is sign-in via magic e-mail link, i.e. let the user enter the e-mail, generate a long random

Easiest and most secure (in the sense of not getting things wrong) is sign-in via magic e-mail link, i.e. let the user enter the e-mail, generate a long random ID (16-32 bytes), send that to the user via e-mail and let them click on the link there to sign in. Of course if you're app integrates with one of the services you mentioned then a social login makes sense, people have different preferences but from my experience for B2B apps most people will want to use an e-mail or sign in via SSO.