It's probably even harder to compare horses of different eras than it is human athletes. The great ones race as three year olds and never again. I would have to say Secretariat is the Michael Jordan of horseracing just because he won by such a large margin in the Belmont and had the fastest time ever. Greatest horse never to win a Triple Crown race was Alydar. Finished a very close second to Affirmed in all three races. Sham was close in the Derby and Preakness but way behind in the Belmont.
