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.
By that standard Ellen White is a candidate too, as the Seventh Day Adventists are also a worldwide phenomenon and comprise a substantial percentage of the population in several countries.
I didn't vote, since I don't think there is really one solid answer to this. American religion is much too diverse for any one figure to be important enough to have sufficient impact to truly merit the title.
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