“The worst writers”—do you think that’s fair?
I’m not familiar enough with the others to judge, but Arthur Conan Doyle’s a pretty good writer. If you want to say he isn’t one of the greats because he lacks depth or complexity or his prose style isn’t carefully wrought, okay. But what he attempted to do, he did well, and purely as an entertainer, he’s one of the best.
To the OP: I think there are at least three distinct levels to writing. (1) There’s the creation or imagination phase, where you come up with a story to tell, and people to tell it about and maybe even a place to set it in. (2) There’s the storytelling—actually telling the story you have so that your audience can understand it, appreciate it, become involved in it. (3) There’s the actual writing, the style, the actual picking out words and crafting sentences and paragraphs. Some successful writers are good at one or two of these but not so hot at another; and different parts matter more to different people.
On preview…
Bad writing in my 2nd and 3rd senses above, but good writing in the 1st sense if a good movie could be made from it.