Secretariat was the greatest race horse that ever lived. He won the Derby, Preakness and Belmont in record time and the Belmont by 31 lengths. No horse has even come close to that performance. Get the movie—great story that you can watch over and over.
I have seen the movie and I agree it's good.
I hedged on calling Secretariat the greatest race horse, not to diminish the unquestioned greatness of Secretariat but because the greatness of some other horses needs to be considered. Also, the idea of "greatest" needs to be defined more carefully.
Best three-year-old? Secretariat would probably get my vote. He didn't run as an adult so it isn't known if he would have continued his dominance over his peers. Secretariat beat Forego as a three-year-old but Forego (after being gelded) went on to become an all-time great adult racer. Could Secretariat have done the same? Perhaps but we'll never know for sure.
Fastest ever? In a race against every horse that ever lived, you wouldn't catch me betting against Secretariat. However, he could be beaten. He was not undefeated when he ran in the TC races and he lost two races after winning the TC.
Success at stud? Secretariat was a disappointment in this regard. On the other hand, the descendants of Man o'War dominated stake racing for decades.
Reputation and cultural impact? Secretariat hasn't reached the levels of Man o'War for sure and probably not Citation. It seems that most ratings with credibility (i.e. done by horse racing people) have Man o'War rated above Secretariat.
All things considered, Secretariat could well be the greatest race horse of all time. Without question, Secretariat is in the top three.