Hubs about things people search for and that give helpful information. Hubs about Hubs don't generally make money because most people looking for those would be from within this site, itself. Writing about something a lot of people search for, but writing something different from "everything else out there" is good. If too many people are writing about the same thing there'll be lots of competition, and your Hub would have to really stand out. Even then, it can take awhile for that to happen.
There's lots of information on this site for anyone who searches for information about writing or SEO stuff. You can lots of solid feedback from experienced earners in the "help" forum too.