Who of these is the greatest American religious figure?

[Blue3]

Roger Williams. From his "liberty of conscience" belief in the early 1600's comes both the ideas of (1)  freedom of religion and (2) separation of church and state, later enshrined in the First Amendment to the Constitution. He also treated the Native Americans as equals, and we was an abolitionist centuries before the Revolution. This spirit of harmony made colonial Rhode Island into a safe haven of "Providence" for outcasts, religious dissidents, and other minorities of all backgrounds (making RI the home of the first synagogue in North America, the first Baptist church, and becoming the most Catholic state). The Pilgrims left the UK to establish their own theocracy. But Roger Williams is the one who made what would become the United States to a safe haven for all people of all religions and backgrounds, what the Statue of Liberty claims this country is.

Does it get more influential than a religious dissident getting his beliefs into the Constitution and enshrined in our most iconic national symbol??
(the only other acceptable answer from this, in my opinion, would be William Penn)

[Blue3]

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.)

[Blue3]

In terms of being my own personal favorite I’d choose Robert Williams for the reasons already stated. In terms of wider significance and success of their work in the modern era, then it definitely is Joseph Smith given the fact that his sect has spread worldwide.

I think Roger Williams still beats Joseph Smith there. The ideas of freedom of religion and separation of church and start have spread wider and deeper than Mormonism.