Of all the Presidential candidates of the 20th century who lost in the general election, who are some that would have been particularly good as President had they won (and maybe even better than their successful opponent)?
I've answered this question or some form of this question many times over the years, so I'm not going to go into details explaining my reasoning behind my answers, but the losing candidates from the 20th Century that I think would have been pretty good Presidents include Charles Evans Hughes (lost to Wilson in 1916), Adlai E. Stevenson (lost to Eisenhower in 1952 and 1956), and Michael Dukakis (lost to George H.W. Bush in 1988; terrible campaigner but an excellent administrator).
Whenever this question comes up, I always can't help adding one person who never ran for office but who I think would have been a tremendous President: General George C. Marshall, who I think is the most underrated giant of the 20th Century. Winston Churchill called Marshall "the noblest Roman of them all" and the "true architect" of the Allied military victory in World War II. The Big Three leaders of the Allies in World War II -- Franklin D. Roosevelt, Winston Churchill, and Joseph Stalin -- all felt that way about Marshall, as did General Dwight D. Eisenhower. FDR and Churchill both believed Marshall was the greatest man they ever knew, and even Stalin once remarked that he would trust General Marshall with his life.