Secretariat. Not only set the track record in the Derby, but ran each quarter faster than the one before. Went on to set track records in all three Triple Crown races (didn't get credit for the Preakness due to clock malfunction). Seattle Slew, only undefeated Triple Crown winner second. But all I can really say is who I think was the best of the modern era. I mean, can you say who was better, Citation or War Admiral? I know I can't.