Ask HN: What is the best account registration funnel/flow you've used, and why?

We're reviewing options, and I'd love to hear what services/what SSO options/what flow seems to be the easiest/best for those who have implemented many for their own companies. We've talked a lot about what fields to use at a minimum, what SSO providers make the most sense to offer, and how to keep things kosher from a security perspective; however, I feel like we've barely scraped the surface. ... so what is the best account registration funnel/flow you've used, and why?