I honestly don't know. I have a model horse of Man o' War and I just find his build amazing. I know Secretariat won a race by a lot of lengths, don't remember how much though.
And I loved the Seabiscuit movie. I thought he really grew into the race Red thought he would be, and he overcame a break that vets thought would end his career. War Admiral was a highly impressive horse too, he just had this presence that really wowed me (both of those are just based off the book and movie, I never really was into horse racing)
I didn't vote because I didn't know which one to choose.