I'm just going to point out a few things that while Shakespeare is probably the most quoted writer of all time, Henry James is probably the most influential writer of all time. Especially as it relates to form. I think when you few it from the greatest single work of all time, guys like Tolstoy, Melville, Joyce, Proust come to mind. Probably my favorite writer is Ralph Ellison or Andre Malraux.
I'm not a big fan of Mark Twain. He was far too cynical of a writer, if you peal back the layers of his stories, he could have been (and was) the Jon Stewart of his day. Incidentally, I'm not a big fan of Jon Stewart either. Hell, I think Stephen Crane had more to say and said more in the Red Badge of Courage than Twain said in all of his works.
Guys like paul auster, tobias wolff, don delillo, james salter and frankly raymond carver would object to that.
another black female writer who gets overlooked because she writes a lot of urban female romance novels is Terry McMillan. People who think that's all she does should read Mama.
There is only one writer who has outsold Shakespeare,,,anyone know? They are the only two who have surpassed $1b, and they are listed somewhere between $2 and $4 billion (depending upon how you measure), putting them 2.5x to 4x higher than the third highest seller. Spoiler Agatha Christie
We aren't talking about the non-fiction list. See, even the Guinness Book of World Records believes God isn't fiction.
Best western civilization writer? If we include the East and South... I still think OP is the greatest.