In my view the greatest flat racehorses in the British Isles in the last 100 years were MillReef, Brigadier Gerard, Nijinsky, Dancing Brave and Shergar.
Nijinsky has the best statistical record in terms of big races but did not have as good opponents as MillReef, who I personally vote the best post-war British Racehorse.
My personal best in the middle distance was MillReef who simply annihilated some of the finest hoses being the only horse to win the Epsom Derby, Eclipse Stakes, King George and Arc.