If by top Churchman you mean high ranking / Papabile, I would nominate my very own Metropolitan, His Eminence Sean Cardinal O’Malley, Metropolitan of the Roman Catholic Archdiocese of Boston.
If you mean most influential and recognizable, I’m inclined to say His Excellency Bishop Robert Barron, Auxillary Bishop of the Roman Catholic Archdiocese of Los Angeles.
And piggy backing on @twf from a Traditional Roman Catholic perspective, the Archdiocese of Baltimore is the pre-eminent episcopal See and enjoys the “prerogative of place” in the USA, so that would mean His Excellency Archbishop William E. Lori would take the cake here.
The Churchman in the United States with the most political power and therefore most temporal power would be the President of the United States Conference of Catholic Bishops (USCCB), which is currently His Eminence Daniel Cardinal DiNardo.
The Churchman with the most number of souls directly under his jurisdiction would be His Excellency Archbishop Jose Gomez, the Ordinary for the Roman Catholic Archdiocese of Los Angeles, who has over 5,000,000 souls that he is responsible for.
Finally as mentioned previously, His Eminence Timothy Cardinal Dolan, Archbishop of the Roman Catholic Archdiocese of New York could certainly be considered the top US Churchman as well, he is highly influential and holds tremendous power temporally and spiritually.
So any of the following could be candidate for top US Churchman - maybe start a poll asking people to vote who they feel is the #1?
Archbishop William E. Lori, Archbishop of the Baltimore Archdiocese
Sean Cardinal O’Malley, Archbishop of the Boston Archdiocese
Timothy Cardinal Dolan, Archbishop of the New York Archdiocese
Archbishop Jose Gomez, Archbishop of the Los Angeles Archdiocese
Bishop Robert Barron, Auxillary Bishop of the Los Angeles Archdiocese
Daniel Cardinal DiNardo, President of the USCCB
I would say the top US Catholic Churchmen is definitely one of these men.
IMO, I would give the title to either Cardinal DiNardo, Cardinal O’Malley, or Cardinal Dolan - I’m not sure which one, though I’m inclinced to lean towards Cardinal DiNardo.