Before the Arlington Million won by John Henry, I owned a share of a horse named Caterman who had just come off beating John Henry in the Sunset Handicap and was prepping for the big race at Del Mar. Unfortunately, he fractured his leg a week before the Million.
I am old enough to have seen almost all the horses listed, and my vote would go to Secretariat. He could move to the lead at will any time from anywhere and take command when asked whether dirt or turf. He still holds the record for both the Kentucky Derby and Belmont Stakes and only because of a questionable clock at Pimlico during the Preakness, does he not hold that record as well.
John Henry, Forego and Affirmed were all heart. The first two carried tremendous weight assignments no longer used in the United States. Affirmed would look you in the eye and not let you pass.
Seattle Slew and Spectacular Bid had speed to spare. Ill fated Ruffian grabbed your heart and went to her grave in a match race with Foolish Pleasure that should have never been run. Tragic. Buried at Belmont. Easy Goer was very good but is not in the class of the others above. As was Ack Ack.
Man O' War, the original Big Red, I never saw but history indicates he may have been close to as good as Secretariat.