Tommy and Jason were the most popular characters when Power Rangers originally aired. Even though the show tried to make it seem each individual Ranger was important in their own way, there was an obvious hierarchy. Despite Tommy and Jason, I am going to suggest that, out of the original rangers, Billy was the most important.
As a fighter, Billy wasn’t the best, but neither was he the worst. Though he started off really poorly, he developed into a phenomenal fighter. Jason, Tommy, Adam, and Rocky all practiced martial arts un-morphed, on top of being martial arts teachers at the youth center.
There are some scenes where we see Billy practicing martial arts, but it doesn’t happen often. In the episode “Blue Ranger Gone Bad”, the evil Billy is seen working out and, to the other rangers, this is seen as odd. This means that most of Billy’s physical development happened during battles.
Near the end of the season 2, Billy’s physical abilities are up to par with the rest of the rangers, but this isn’t his only skill. His distinguishing feature is his intelligence. In the second episode, he had already invented a way to keep in touch with Alpha and Zordon. In this case, Billy frequently came up with new ideas to help innovate the rangers, and even after he retired as a ranger, he helped Alpha and Zordon deep into the season.
Billy is the perfect mix of Zordon/Alpha and the Power Ranger team. That is why he is the Ultimate Mighty Morphin’ Power Ranger.