What do you know about the playing strength of an all-time great super-GM? That he can see all sorts of crazy subtle stuff that you can't, but you do understand it after it is too late? Me, too.
Anyway, I find it quite interesting that the Morphy detractors in this thread think that playing strength has very little bearing on the ability to judge another player's... uhhh... playing strength. If opinions of stronger players carry any weight, and for that reason you do not want to take my opinions seriously, then you can take those of the former world champions that BlunderLots quoted.
If you don't believe them, then I invite you to believe me, because I am quite possibly 3200 strength in theoretical discussions that can never be proven.
Dominance is dominance in any competitive field.
Regardless, I would like to hear more from other opinionated, but strong, players (not the politically correct players afraid to voice a strong opinion).
I think this is where the advances in opening theory cloud our judgments regarding chess skill.
I believe a modern stable 2400 player with a complete sound opening repertoire and who has memorized the entire ECO would get thoroughly routed by Morphy in a match.
I didn't say what would happen if he played a 2400. I'm saying IF a 2400 went back in time, you wouldn't be able to tell the difference. I'm saying it's not possible to judge Morphy as the best when his opponents weren't so bad (relatively speaking).
And I think people misunderstand when they talk about opening theory as if that's what makes people good. All of chess has advanced. Opening, middlegame, endgame, attack, defense, and all sorts of strategic concepts.