What makes an author great to you?

I'm just curious what you guys think. Does an author become great when they help shape a generation of readers (love her or hate her but jk Rowling comes to mind). Or is it the influence over the whole genre such as tolkien or GRRM? (Even with ASOIAF not having been finished). I know some people who love Sanderson for the story but also because he's active and consistent. Or is it based solely off the number of books the author can get you to read from them? (My most read authors are Rick Riordan and David dalglish)