In terms of themes, one of the more interesting ideas I see in literature across cultures and across time periods is the relationship human beings have with nature. Although some cultures and authors view the relationship more positively or more negatively, it always seems to be present.
It's hard to disagree with the previous posts that name William Shakespeare as the most influential writer ever. He is certainly one of my favorites. As far as American writers go, Edgar Allan Poe is my favorite. I especially love short stories, and his are some of the greatest mysteries ever written. He obviously had great poetic skills as well. Other favorites are William Faulkner, John Steinbeck and Ernest Hemingway. Another of my favorites who never reached the higher echelons of fiction writing was Jerzy Kosinski, a Polish-born writer who wrote beautifully in his second language, English.
I must concur with ask996, for Shakespeare is an absolute genius; his comprehension of the human being is amazing. The erudite and renowned critic Harold Bloom contends that it is Shakespeare who teaches Freud and all other psychologists. Indeed!
While many of us love reading Shakespeare, it is truly impossible to limit ourselves to one author. So many other names come to mind--Charles Dickens, the Bronte sisters, Victor Hugo....For me,Thomas Hardy is at the forefront, as he touches readers emotionally (Tess of the d'Ubervilles) and his beautiful prose is sorrowfully poetic. Poets such as Keats, Wordsworth, Dickinson, Whitman, Hardy, Wallace Stevens..... It seems our temperaments relate to authors of similar ones, so for us they are great.
Other editors will contribute more great names.
William Shakespeare would have to be one of the writers at the top of my list. His poetry is beautiful, his plays offer a wide variety of subject matter, and his use of the English language is masterful. What other writer has been able to make up words that become assimilated into usage? What other writer has attained such widespread longevity?