Probably the strongest player never to become US Champion was Rueben Fine, who tied for first at the 1938 A.V.R.O. tournament, and had a winning record against Alexander Alekhine, but retired early to become a psychiatrist.
Despite his international success, he never managed to become US Champion, falling behind Sammy Reshevsky in 1936, 1938, and 1940. After failing to win a superior last round game against Reshevsky in 1940, he never tried again while Sammy was around. But he did make one more attempt, in the fairly weak 1944 Championship, only to have his 14½-2½ score beaten by Arnold Denker's 15½-1½. This game between them, from Round 6 ended up deciding first place.
Notes by Denker, except those prefaced with "(NOTE:)".