Underappreciated philosophers active in the U.S. from roughly 1900 through mid-century?

So with 260 votes, here are the results:

1. Clarence Irving Lewis  (Condorcet winner: wins contests with all other choices)

2. Alfred North Whitehead  loses to Clarence Irving Lewis by 99–80

3. George Santayana  loses to Clarence Irving Lewis by 111–53, loses to Alfred North Whitehead by 97–51

4. George Herbert Mead  loses to Clarence Irving Lewis by 106–46, loses to George Santayana by 66–62

5. Susanne Langer  loses to Clarence Irving Lewis by 104–40, loses to George Herbert Mead by 54–52

6. Tied: Brand Blanshard  loses to Clarence Irving Lewis by 109–33, loses to Susanne Langer by 57–51 Roy Wood Sellars  loses to Clarence Irving Lewis by 109–23, loses to Susanne Langer by 50–44

8. John N. Findlay  loses to Clarence Irving Lewis by 111–31, loses to Brand Blanshard by 50–47

9. Charles Hartshorne  loses to Clarence Irving Lewis by 109–31, loses to John N. Findlay by 46–41

10. O.K. Bouwsma  loses to Clarence Irving Lewis by 111–26, loses to Charles Hartshorne by 54–39

11. C.J. Ducasse  loses to Clarence Irving Lewis by 107–17, loses to O.K. Bouwsma by 47–34

12. Monroe Beardsley  loses to Clarence Irving Lewis by 117–27, loses to C.J. Ducasse by 44–42

13. Maurice Mandelbaum  loses to Clarence Irving Lewis by 111–24, loses to Monroe Beardsley by 44–38

14. Ralph Barton Perry  loses to Clarence Irving Lewis by 114–16, loses to Maurice Mandelbaum by 41–34

15. Tied: Alain Locke  loses to Clarence Irving Lewis by 111–18, loses to Ralph Barton Perry by 35–30 Walter Stace  loses to Clarence Irving Lewis by 117–11, loses to Ralph Barton Perry by 37–25 Paul Weiss  loses to Clarence Irving Lewis by 110–18, loses to Ralph Barton Perry by 38–29

A couple of readers pointed out a significant omission from the survey, Morris R. Cohen.  Feel free to list other omissions in the comments, but take a moment to say why they were significant and should be appreciated today.  More generally, I'd be keen to hear from readers what they think of the philosophers on this list, why they are significant and still worth reading.  That will surely be more interesting than the mere ordinaly listing.