Who were the most famous writers during the American Romantic era?

The most famous Romantic writers in America were probably Emerson and Thoreau; among the Dark Romantic writers, Hawthorne, Melville, and Poe are renowned.
Well-suited to American democracy and expansion, the Roman spirit affirmed the value of every man, and expressed the inspired imagination of ethical and aesthetic values. A close examination and development of self, therefore, became a trope for the American Romantic writers. According to the theories of Romanticism, since self and nature are one, it is not selfish to be self-aware; instead, this cognizance is a "mode of knowledge" that assists in the explanation of all that surrounds man.
No one expressed the concepts of Romanticism better than Ralph Waldo Emerson. His love of nature and his sense of the sublime were aptly expressed in his work and his convictions about the value of the individual and the importance of self-expression and self-reliance. Another advocate of individualism was Henry David Thoreau, who urged men to "march to the beat of a different drummer."
Often called the greatest American Romantic poet, Walt Whitman developed a style of poetry that was uniquely democratic and American in its structure. Employing simple language so the public could easily access his poetry, Whitman wrote of the unity and equality of Americans:

I hear America singing. . .The day what belongs to the day — at night the party of young fellows, robust, friendly, Singing with open mouths their strong melodious songs

Emily Dickinson, too, examined the American heart and mind, expressing freely individualistic ideas, ideas that for her were liberating.
Other famous writers of the Romantic period in America are Washington Irving, Nathaniel Hawthorne, Herman Melville, and Edgar Allan Poe.
Hawthorne, Melville, and Poe are often categorized as Dark Romantics, writers who acknowledged the existence of evil in human life. In fact, they formed a counterpoint to the Transcendental optimism of Emerson and Thoreau.