Capablanca's statement on Morphy for an Uruguayan chess magazine in 1927:
Morphy was a great stylist. In the opening he aimed to develop all his pieces rapidly. Developing them and quickly bringing them into action was his idea. In this sense, from the point of view of style, he was completely correct. In his time the question of Position was not properly understood, except by himself. This brought him enormous advantages, and he deserves nothing but praise. It could be said of him that he was the forerunner of developments in this extremely important part of the game. He made a special study of the openings, with such success that in many games his opponents had an inferior position after six moves. This is also praiseworthy since in those days he had little to guide him. Players of the time thought that violent attacks against the king and other combinations of this kind were the only things worthy of consideration. It may be said that they began by making combinations from the first move, without paying sufficient attention to the question of development, about which Morphy was extremely careful. His games show that he had an outstanding playing style. It was simple and direct, without affectation; he did not seek complications but nor did he avoid them, which is the real way to play. He was a good endgame player and proved himself a clever defender of difficult positions. His combinative power was wholly sufficient for what he undertook, but it was not, as most players of today think, the most important aspect of his talent. That was his style, which, as far as could be judged, was perfect.
Morphy was probably the all time best relative to his peers but it's hard to judge since the game is always developing and the bar is always set higher. I think Morphy is probably more impressive since he did not have any contemporary rivals to push him to become better and yet he still ascended to a level never seen before, while Capablanca had players like Marshall, Lasker, and Alekhine to test himself against and to push him to better results.
As Lasker said: There was only one chess genius: Capablanca.
Morphy beating up a Duke at the opera is spectacular, but not the same as prevailing in matches and tournaments against the best.