Do you mean writers currently active now, or just my favorite writer of all time? And are we talking fiction or non-fiction? Ah, too many choices!
In terms of contemporary fiction writers, it is hard to keep up, because I devote roughly a third of reading to fiction and the other two-thirds to non-fiction, which means I'm constantly playing a game of catchup when it comes to fiction. Kazuo Ishiguro is an obvious choice, but few others spring to mind. It's not because there aren't other great authors, but the immense literary output of the 21st century and my own inability to keep up makes it hard to stay up on current trends. I have been impressed by what little I have read by Jeff VanderMeer and Brian Evenson, but as I said, it has been little; I only read the first book of the former's Southern Reach trilogy.
If we're talking all-time fiction greats, Ishiguro reaches this list as well, as does Cormac McCarthy and Neil Gaiman. For essay-style non-fiction writing, Christopher Hitchens, Barbara Ehrenreich, James Baldwin, and George Orwell are tops (and the latter two's fiction is also excellent). For history, Theodore Draper, Gordon S. Wood, Jonathan Israel, and David Harvey (okay, technically an anthropologist, but it would feel remiss not including him).