I choose Walt Whitman because of his unique voice, his very American viewpoint, and his influence on future poets:  Carl Sandburg, Allen Ginsberg, etc.  He brought his whole self to his poetry, with almost no regard for commercialism.  If Art occurs twice--once in the creation and once in the reception--Whitman is a true Noth American Artist.

I think if you are looking for definitive criteria for great contributions of American Literature (and Canadian) to English Literature, I'd suggest you start with Pulitzer Prize winners. Faulkner won the 1955 and 1962 Pulitzer Prizes for Fiction for his novel A Fable (1954) and for the Yoknapatawpha novel The Town (1957). This says that the Literary and critical experts of the day deemed Faulkner's a great American contribution to English Literature. This was a judgement backed up with prize money of USD $7,500, so it is judgement by experts, not opinion. Another American who won the prestigious Pulitzer for Fiction is Ernest Hemingway in 1953 for The Old Man and the Sea (1952). John Steinbeck won the 1940 Prize for The Grapes of Wrath (1939). American John Updike won in 1991 for Rabbit at Rest (1990) and earlier in 1982 for Rabbit is Rich (1981).

So if (1) Pulitzer Prize receipt and (2) longevity of a work can be the foundations for judgement on contributions to English Literature, these American authors can be said to have made a contribution to English Literature. Yet this is where opinion kicks back in: Does one agree that Faulkner's or Steinbeck's or Hemingway's or Updike's work should have been awarded the Prize, and does one think that the contribution made was a beneficial one? There are those who would answer a resounding "Yes!" while there are those, like psychologist Peter Kramer who authored Against Depression, who would answer resoundingly "No!" Yet if you want factual support for contribution, the Pulitzer is pretty hard to ignore.

Continuing on the themes of horror fiction, H.P. Lovecraft was an immense influence during the early 20th century. His ideas of Cosmic Horror had been alluded to but never codified; he all-but created the genre of "weird" fiction, melding elements of straight horror with science-fiction and the supernatural. Stephen King cites Lovecraft as influential, as do many modern-day horror writers.

The concept of the "Great American Novel" changes, it seems, week-to-week, but most critics agree that the introspective novels of the early 20th century are the most important and influential. The Great Gatsby worked on multiples levels, being a romance, a thriller, and a satire on the Jazz Age. The Catcher in the Rye became massively-read through controversy, but is considered a defining work of the 1950s.

I will personally cast my vote for Mark Twain, though, as his experimental works in realism, fantasy, and short stories have influenced and continued to influence writers throughout the last century.

Edgar Allan Poe's writing made a great change in literature. The short story became a genuine genre with his classic stories.  As the first American mystery story writer and with a detective, his story "The Murder at the Rue Morgue" began a totally new type of story. From the length of his stories to the subject, Poe's writing was unique. 

Broaching subjects like murder, perverseness, revenge, psychological problems--these were the new exciting stories that Poe churned out.  During his short lifetime, he was not appreciated; after his death, he was considered the father of the short story. 

His stories are considered classics and should be read by any English speaking person.  "The Cask of Amontillado" is considered by many literary critics as an almost perfect short story.  But the others are equally intriguing: "The Black Cat," "The Tell-Tale Heart," "The Masque of the Red Death," and many others.  

Walt Whitman, the great American poet, labeled Poe as a genius and his poetry as beautifully written. Certainly, many of Poe's are considered to be classics and some of the best know poems in American literature.

George Lippard, a writer and critic for a Philadephia newspaper wrote about Poe:

Poe 's mind is stamped with the impress of genius. He is, perhaps, the most original writer that ever existed in America. Delighting in the wild and visionary, his mind penetrates the inmost recesses of the human soul, creating vast and magnificent dreams, eloquent fancies and terrible mysteries.

Poe's poem "The Raven" stands alone as a classic verse and accepted as one of the great American poems.  His other poems added to his accreditation as a first class poet: "To Helen," "The Bells," "Annabelle Lee."

When is a contribution to literature considered great? Writing has to be changed forever.  With his unique approach to the short story, Poe's name will go down as the first mystery and horror story writer. Stephen King can thank Mr. Poe.

The poetry of T.S. Eliot was definitive (or at least representative) of a big moment in western literature. Though Eliot is sometimes associated with England, he was American born and his work is expressive of a metaphorical break growing between the "old world" and the new. 

From the use of multiple languages to the heavy use of allusion along with narrative, lyrical and liturgical forms marked a watershed moment in literature. This moment included Hemingway and Faulkner and Pound - other Americans - and was characterized by a sense that modernity was wiping out antiquity, tradition, and possibly culture too. 

If you are referring to literature written in North American in the English language, I would have to choose The Adventures of Huckleberry Finn by Mark Twain.  It has often been said that American literature began with this book.  It definitely was influential.  It embodies the American spirit of adventure, willpower, and conscience. 

One of the most important most memorable scenes in the play is when Huck is trying to decide if his feelings about slavery is right, and society is wrong.  He starts to write a note to Miss Watson telling her where her runaway slave Jim is, but he feels bad.  He feels bad because he does not want to tell on Jim.  He thinks he will be committing a sin if he does not turn him in.

I was a-trembling, because I'd got to decide, forever, betwixt two things, and I knowed it. I studied a minute, sort of holding my breath, and then says to myself:

“All right, then, I'll go to hell”—and tore it up. (Ch 31, p. 141)

This one line is one of the most powerful in all American literature, because it suggests the American individualism and moral imperative.  Huck had to choose between two things, and he knew that choice would change him forever—but he chose to follow his conscience.

First, we should set some criteria--most influential?  Most copied?  Most cited?  Most "American"?  And does Noth America include Canada?  Because I'm tempted to nominate Leonard Cohen.  But for now, I nominate Walt Whitman's Leaves of Grass.