As far as I know, the only WWII "War Criminal" who actually served out his entire sentence (without suffering a heart attack or falling to disease, taking his own life, "falling in the shower" at the hands of a guard, etc) was Rudolph Hess.
The fact of the matter is that many of the war criminals were given leniency or clemency for a variety of reasons, primarily amongst them because the Allied War Crimes Tribunal really had no (international)legal standing to stage trials or hand out sentences in the first place, because some (by no means all) were falsely accused or had dubious evidence arrayed against them, or because of the post-War power shift (West vs. USSR). Many were used in or near their old capacities in the new governments of West Germany and Japan. In this capacity, many German and Japanese military leaders avoided the gallows or a life sentence because their experience was required to rebuild the governments and armed forces of these countries quickly and efficiently,or because they had particularly useful skills and information.
The best examples I can give of this last phenomenon would be Reinhardt(?) Gehlen, Werner Von Braun, Karl Doenitz, and the Japanese scientists who conducted germ warfare experiments in China and Manchuria. Quite frankly, the idea of a War Crimes Tribunal of any kind strikes me as patently ridiculous, especially when it's staged and enforced by triumphant Western nations who invented the concept of the Total War of Annhiliation, and who themselves had engaged in many of the same activities as the accused (extra-judicial execution of prisoners in violation of the Geneva Conventions, indiscriminate bombing of civilian populations, unrestricted submarine warfare, use of proscribed weapons on the battlefield, etc).