Secretariat winning the Belmont stakes by 31 lengths to win the triple crown. Come on now, who wouldn't???!!!! Riding a part of history. And the fictional horse I would want to ride would be Pegasus, for obvious reasons, lol.
As fictional horses go Sleipnir the Norse God Odin's 8 legged horse. Real horses Sea Biscuit, Man-o-War or a young Three Bars.