Shatner hit a grand slam in Where No Man Has Gone Before, which is even more impressive considering this was his first time playing Kirk.
..which is why he was the superior performer to lead the series over someone like Jeffrey Hunter (or any of the other names often cited as possible casting choices). A significant part of the success of "Where No Man Has Gone Before" (and one of the reasons it sold Star Trek as a series) was Shatner's performance in a story taking him from friend, leader, troubled and enemy all in one episode, and he delivered a wonderful performance. He was tough, adverturous, but all so human, establishing Kirk as a partially tragic character, which would be explored in other episodes (e.g., the back-to-back punch of "City on the Edge of Forever" and "Operation -- Annihilate!"), but it was in WNMHGB that Shatner made an unforgettable mark as Kirk.