I think the most open (and perhaps "easiest for users") is still just using email and password. It's the baseline, and unless social features or real-world identity verification is a core part, email/password is available as a signup method.

It's also open (two text strings), decentralized (you can use whatever password manager you want), and simple (people have been using it for ages). So before implementing anything else fancy, I'd highly recommend doing email/password first.