The question "Who is the most prolific designer/sculptor of classic-era US commemorative coins?" is not as easy to answer as one might expect. Part of the reason for this is the fact that the question is open to interpretation.
Is the question "Who was hired the most times to model a US commemorative coin?" or is it "Who created the most designs when working alone?" or is it "Who modeled the most commemorative coins regardless of circumstances?" or one of many other variations?
It should be remembered that US commemorative coins are generally fund raising vehicles for their sponsoring organization. As such, these organizations invariably offer design suggestions to whichever artist is selected to create the coin's designs/models.
In the early days of the classic era of US commemorative coins, the design and creation of the needed plaster models generally fell to the engraving team at the US Mint, with
Charles E. Barber and George T. Morgan leading the way.
For example,
Charles Barber is acknowledged as having had his hand in the development of nine different classic-era US commemorative coin design types. George Morgan was involved with seven coins. In some cases, Morgan assisted Barber with a given coin (e.g., 1892-93 World's Columbian Exposition Half Dollar). In multiple cases, each of these designers/engravers adapted the previous designs of others (often from medals) for use on a US commemorative coin.
Should either of these US Mint employees be considered the "Most Prolific"? To me, the answer is "No!" IMO, they relied too much on the work of others to be considered the true "designers" of many of the coins on which they worked. I liken it to the relationship between an author and editor. A good editor can greatly refine an author's work and make it better, but I would rarely give them credit as an author for their contributions.
For example, though Barber and Morgan share credit for the 1892-93 World's Columbian Exposition Half Dollar, the coin's designs are based on the uncredited models of Olin Warner. (See
1892-93 World's Columbian Exposition - U. S. J. Dunbar & Olin Warner for more.)
So, where does that leave us?
Multiple artists are linked to three classic-era US commemorative coin designs (alphabetically by surname):
Robert Aitken has three classic-era US commemorative coin credits: the 1915 Panama-Pacific International Exposition Gold Quintuple Eagles ($50) - both varieties, the 1921 Missouri Statehood Centennial Half Dollar and the 1935-1936 California-Pacific International Exposition ("San Diego") Half Dollars.
Charles Keck is credited with the designs on the 1915 Panama-Pacific International Exposition Gold Dollar, the 1925 Battle of Bennington/Vermont Independence Sesquicentennial Half Dollar and the 1936 Lynchburg, VA 150th Anniversary Half Dollar.
Henry Kreis developed the designs for the 1935 Connecticut Tercentenary Half Dollar, the 1936 Arkansas Statehood Centennial, Robinson Type Half Dollar and the 1936 Bridgeport, CT Centennial Half Dollar. (Kreis based the Arkansas coin's portrait of Senator Joseph Robinson on a drawing provided by Enid Bell.)
John Sinnock is associated with the reverse of the 1918 Illinois Statehood Centennial Half Dollar and the two 1926 American Independence Sesquicentennial coins - the Half Dollar and the Gold Quarter Eagle ($2,50). (Sinnock created the half dollar designs from ideas/concepts presented by John Frederick Lewis.)
William Marks Simpson is another artist with three credits: the 1937 Battle of Antietam 75th Anniversary Half Dollar, the 1937 Roanoke Colony Memorial Half Dollar and the 1936 Norfolk, VA Bicentennial/Tricentennial Half Dollar. (Simpson worked on the Norfolk coin designs with his wife Marjorie Emory Simpson.)
There are also a pair of artists that were involved with four coin designs:
Chester Beach is linked to four US commemorative coins: the 1923 Monroe Doctrine Centennial Half Dollar, the 1925 Battle of Lexington-Concord Sesquicentennial Half Dollar, the 1928 Hawaiian (European) Discovery Sesquicentennial Half Dollar and the 1935 Hudson, NY Sesquicentennial Half Dollar. (Beach reworked the designs of Juliette May Fraser for the Hawaiian coin, so should not receive full "designer" credit, IMO.)
Laura Gardin Fraser also has credits on four classic-era US commemorative coin designs (though credits are shared on two): the 1921 Alabama Statehood Centennial Half Dollar, the 1922 Grant Birth Centenary coins (Gold Dollar and Silver Half Dollar - common design), the 1925 Fort Vancouver Centennial Half Dollar (reworked designs of Sidney Bell) and the 1926-39 Oregon Trail Memorial Half Dollar (a collaborative project with her husband
James Earle Fraser).
So, who should be considered the most prolific?
Pushing aside any thoughts of "favored design" or total coins issued and focusing only on volume of design work produced, I would suggest the crown should go to either Robert Aitken or Charles Keck based on their individual and non-derivative work. Of the two, I'm going to give the nod to Robert Aitken, as he also designed the US Mint-struck commemorative medals for the 1915 Panama-Pacific International Exposition. Yes, Keck designed a medal for the 1927 Vermont Sesquicentennial, but the US Mint connection for Aitken's medals and their direct parallel to the Exposition coins pushes him to the top by a half a tick.
Of course, that's just my opinion. Your mileage may vary.

For more of my posts about commemorative coins and medals, including others about each of the coins (and medals) mentioned in this post, see:
Commems Collection.