What websites have the best new user onboarding flows?
When it comes to on-boarding new users in a web site or app, there are two ways: the old way, and the right way.
The old way was akin to the setup processes that we see when you install a new or upgraded operating system: a stepped approach that walked users through the options that were available, so they can customize their experience. The new (and right) way involves asking only the information that is absolutely necessary, and no more, and providing intelligent defaults that the user can change AFTER they are setup. The more steps that you make users do, the more likely they are to abandon the process and you lose the opportunity to have them as a customer.
We have Web 2.0 to thank for this revised, optimized process – there is so much competition for "eyeballs" that the quicker you get users inboard, the better. Mobile has only accelerated that process (with apps like Instagram asking only for a username and password). Now, just because you get quick sign-on/registration doesn't mean you'll get usage and advocacy – the whole experience has to provide value to your users.
Specific examples of good onboarding processes: The aforementioned Instagram, Dropbox, Twitter, and Google.