It makes me sad that in 2018 that it is entirely reasonable for such a simple and common question to elicit so many answers. Of course no one solution fits all use cases, but skimming the comments there seems to be a very diverse range of suggestions. Wouldn't it be lovely if there was one stand-out solution that was so good it was a no-brainer?

FWIW I have ended up using OAuth2 for this situation a few times, and it always feels more complicated than I'd like.