I suppose the Showa emporer would be the most obvious choice, as he reigned for 63 years during which Japan went from a fascist military powerhouse to a war devastated moonscape and then back to being a major world economic power. His career would offer a very broad scope of Japanese political developments during the 20th century, but he wasn't a real leader in any sense, just a figurehead.
I doubt the emperor himself had really a lot to say. Formally yes, but in reality?
Anyway, some names would be Yoshida Shigeru and Tanaka Kakuei, both postwar PMs. The real leaders are behind the screnes though, receiving a nice paycheck from their own LDP party or those closely working with them and standing at the top of the keiretsu...
Yoshida Shigeru is a very interesting person to write about. He was the political leader of Japan twice, with three cabinets. The first Yoshida cabinet (1946-47), many reforms were adopted. During the second Yoshida cabinet, (1948-49) and third cabinet 1949-54), much happened which is worth writing about in a history paper: the Korean war broke out in 1950 which helped revive the Japanese economy, the occupation of Japan ended in 1952 (1951 treaty signed in San Francisco). The peace treaty settlement itself was a cardinal objective for Yoshida.