Regular login/pass will be your lowest common denominator. I think you have to build that first, and add in facebook/google/twitter afterwards. I'd sign up via either google or via login, but if you didn't offer those options I would not sign up.
My reasons are that I have concerns about the long term outlook for my twitter (potentially frivolous) and facebook (privacy issues) accounts. In fact, I've deleted my facebook. So I don't use either of those to tie accounts together, because it locks me further into the service.