...the men who made Nihon Kaigun the most feared naval force in the Pacific, for a time anyway. 

family name last, given name first.

Mastermind of the Pearl Harbor attack, and leader of Combined Fleet until 1943, his plane was shot down over Bougainville on April 18, 1943 by American P-38 aircraft. I personally think Yamamoto was a gifted individual, but also a bit over-rated. His needlessly complex operational scheme at the Battle of Midway dispersed his forces in the face of a still-dangerous foe, and directly led to the disaster there. Further, in the subsequent battles around the Solomons he was seemingly unable to capitalize on the weaknesses of the U.S. carrier forces opposing him, and was also unwilling (until it was too late) to risk sending the heavy surface units of the Combined Fleet down there to kick some butt. This unwillingness, to me, belies Yamamoto's underlying belief that battleships still were the ultimate arm of decision, and should therefore be hoarded in hopes of participating in a Decisive Battle.

 Chuichi Nagumo

Every IJN fan's favorite whipping-boy admiral, and hey, why should I be any different? Gruff, conservative, not overly-imaginative, and at his core profoundly suspicious of the potency of carrier-based airpower, Nagumo was (ironically enough) placed in charge of the most powerful naval air armada of the first half of the war: Kido Butai, Japan's carrier strike force. He was in charge of executing Yamamoto's plan for a Pearl Harbor attack, which was also ironic, because he and Yamamoto got along very poorly, and Nagumo didn't believe in Yamamoto's plan at all. The result; a spectacular, but somewhat superficial victory in Hawaii, wherein the US heavy surface units were largely destroyed or disabled, but our escort vessels and (much more important) our oil tanks farms and repair facilities escaped practically untouched (despite the urging of his staff officers to launch a second attack and destroy them). This, in turn, left the US Navy with both the carrier escorts and the logistical underpinnings necessary to carry on the war.
After Pearl Harbor, Nagumo commanded Kido Butai through its subsequent six-month long reign of terror, during which it roamed the Pacific with seeming invincibility, until it was finally dismembered at the Battle of Midway. Thereafter he went on to lead the Combined Fleet's carrier aviation forces through a series of uninspiring performances around the Solomons, finally being removed from command after the Battle of Santa Cruz, October 25-27, 1942. He died by his own hand during the invasion of Saipan in July, 1944.