I remember reading that during TOS pre-production era GR offered the part of Captain Kirk to Jack Lord, aka Steve McGarrett of his later "Hawaii Five-O" fame. Lord wanted 50% ownership of the show so he was quickly dismissed from the competition. 40+ years after the show aired it's difficult to imagine anyone but William Shatner as Kirk. What's your opinion? Could Lord have filled the starfleet boots as well as Shatner?
