Antoninus Pius
German historian Ernst Kornemann, in his Römische Geschichte (roman history) [2 vols., ed. by H. Bengtson, Stuttgart 1954], said that the reign of Antoninus comprised "a succession of grossly wasted opportunities", given the upheavals that were to come. There is more to this argument, given that the Parthians in the East were themselves soon to make no small amount of mischief after Antoninus' passing. Kornemann's brief is that Antoninus might have waged preventive wars to head off these outsiders.