Twain was once voted the Quintessential American Author. Others who have been listed at his level during our 19th century were: Nathaniel Hawthorne Edgar Allan Poe Stephen Crane Bret Harte Washington Irving and a young man who was inspired totally by him: Jack London.
I would be remiss if I did not point out that one of his contemporaries (and just as much a curmudgeon) and he had an ongoing battle of words in their respective newspapers, Ambrose Bierce...