Up until a few days ago, I would have named Gene Wolfe among very few writers whom I would consider 'the greatest living.'
He was a genius at characterization and his plots were dense and intriguing with layers of implication pertaining to the activity of his stories, as well as layers of meaning.
He wrote Science Fiction and Fantasy but was often compared to Kafka, Nabokov and Borges.
He changed the way I thought about plot, characterization, fiction and writing in general. He will be greatly missed and will always remain an inspiration and an influence.
If you're not familiar with his work, he is best known for Book of the New Sun.
You might read that, but some of my favorites are Wizard Knight, An Evil Guest, There Are Doors, Peace and Live Free Live.