L. Ron feels like the most uniquely American, followed closely by Brigham Young
Scientology isn't nearly as important or influential to the United States as the First Amendment. Neither is the Church of LDS, honestly (from an objective perspective... who knows, maybe the Mormons are right theologically, but Smith and Young are still not the most important religious figures to the United States on this list.)
Williams was a Puritan, born in England and died in a British colony; he was also very not-racist by the standards of his day. Compare to Brigham Young, who believed in a very different religion that said Jesus came to America, tried to create his own country in America and ended up fighting a war against the federal government (nothing more American than violent secession tbh), and was also very racist.
The sci-fi huckster who makes up a cult to litigate a copyright dispute is also a true American archetype. Building up a quasi-military militia and attempting to take over local government in the furtherance of your religious goals is also a strong tradition. It's possible to see a line from Hubbard to the Kansas goat-gland doctor to Wild Wild Country. Nothing more American than your own Netflix special.