What were his times and what was the field against which he ran?
This is an argument akin to who was the greatest pro quarterback of all time. It’s an opinion thing, with no right and no wrong. Personally, I think Joe Montana was the greatest pro quarterbacki. But, what about Slinging Sammy Baugh? What about Norm Van Brocklin? What about Bart Starr? It’s a fun tavern topic.
I doubt anyone knows. I'm not sure they recorded times back then, because the time-pieces then were not terribly reliable anyway. It's unclear that times would have given much information for comparison anyway because they didn't run on beautifully-groomed racetracks back then; the "track" at Ascot was a long green expanse with hills, etc. And the horses were shod much differently than they are today, as can be seen from a glance at the equine portraits of the period. Saddlery, and thus probably weight, was different. They were smaller horses, with the Arab in them close-up (within a generation or three) and they were ridden by fifty-year-old guys in long stirrup leathers instead of young Hispanics who weigh less than 115 pounds at worst. So it's apples and oranges.
Interestingly, the skeleton of Eclipse was analyzed recently to see if it revealed any major conformational differences that would explain his amazing superiority to every other race-horse of his generation. No major differences were found. He doesn't look extraordinary in his pictures, either. At his death Eclipse's heart was supposed to weigh thirteen pounds, much larger than the average horse's heart. In this way he was similar to both Secretariat and Man O'War, who both had great hearts to pump oxygen through their muscles.
This is an argument akin to who was the greatest pro quarterback of all time. . . It’s a fun tavern topic.
Yup, it's fun. We all tend to vote for the horse we saw, and of course I saw Secretariat run, while the guy who saw Man O'War might think he was the superior horse. No one alive now could have seen both run, but perhaps we have a memoir somewhere from a person now deceased who saw them both run and could make comparisons.