Egads, what a difficult question. A few above have mentioned Cormac McCarthy, and I tend to agree, as his writing style is so unique, and his ability to tell a story masterful. Stephen King has many of the same qualities and they both examine the darker sides of human nature and existence, which appeals to me in terms of literary taste. On the less well known side, Rudolfo Anaya, author of such classics as Bless Me, Ultima and Albuquerque makes my list as well.
Don't forget about Maya Angelou! She read her own "On the Pulse of Morning" at Clinton's inauguration and wrote the famous I Know Why the Caged Bird Sings, often considered a work of autobiographical fiction: full of feminism, full of life.
Another famous lady author to consider is Annie Dillard, the modern Transcendentalist. My favorite works of hers are Pilgrim at Tinker Creek and The Maytrees. I've always thought of her as a way for Emerson and Thoreau to live on.
It's difficult to discount Harold Bloom's famous declaration that the four greatest living authors are McCarthy, Pynchon, Roth, and DeLillo. I agree with earlier responses that Atwood, Morrison, and Tim O'Brien are extremely talented. I'm surprised to see that no one has mentioned Michael Chabon, Salmon Rushie, or Jonathan Franzen yet.
I'm a big fan of Murakami, but I don't think of him as being on the same level as the others. However, in the spirit of "totally different directions," I'll mention Neal Stephenson and William Gibson.
Just to push the conversation in a totally different direction, I'd add Haruki Murakami to the list. He is a japanese author and translator. His books, like Hard-Boiled Wonderland and the End of the World, are strange but very well written. I suppose he was the first one to pop into my head because I can think of no other works that can be compared with his.
I know that she only wrote one novel, but Harper Lee--the author of the classic To Kill a Mockingbird--is still alive and in her 90s. Another of my oldies but goodies is Herman Wouk (I love Don't Stop the Carnival and Youngblood Hawke)--I believe he is now 96. I would have to second Cormack McCarthy as well.