Definetly RICHARD RETI !!
This is the man who above all seeked THE BEAUTY in chess. The victory was of less value to him... This man truly LOVED the game of chess - and often missed the opportunity to win in order to explore some new possibilities.
He always tended to go bexond the limits of its time. He bravely stood against the old school (which was represented my great Em. Lasker) and paved the way to hypermodern theory. He also was the best blind chess player ever (braking Alekhine's record with 29 simultanous blind games). He was also famous author of chess problems - you can hardly find a chess book that doesn't contain some of Reti's problems.
His contibution to the chess theory is priceless - in the way, it saved the game of chess from "theoretical draw".
He was a truly scientist of chess, a theoretician. Maybe this and his seek for everlasting beauty in chess prevented him to win the World champion title. And the fact that he had lived in the times when Lasker, Capablanca, Rubinstein and Alekhine were in top shape...
Too bad he never had chance to challenge Capablanca in the Wold champion match... he would probably lose but that games will be real beauties, I'm sure.