I included Khosrow II as an option because one could view him a bit like Hannibal or Napoleon, a spectacular conqueror whose efforts ultimately unraveled, but were quite the effort. But yes, I favour Shapur I and Khosrow I.
Shapur was clearly a great military leader: His victories at Misiche, Barbalissos and Edessa, his capture of Valerian, his army's likely role in the mortal wounding of Gordian III, twice sacking Antioch, raiding as far as Cilicia and Cappadocia, the capture of various cities and fortresses in Upper Mesopotamia (most notably Dura Europos), his conquest of Armenia and Hatra, his victories over Sakas, Kushans and the 'Medes of the Mountains', and his involvement in the Battle of Hormizdagan. That said, there were limits to how long his feudal armies could remain in the field and how much he could therefore actually conquer as opposed to raid beyond the lands of Upper Mesopotamia and Armenia. This limitation of the Sasanian Empire was fixed by Khosrow I.
Khosrow I was not as brilliant in the field, but he was a fantastic ruler. He professionalized the Sasanian army, created a class of lower nobility (the Deghan) to expand the ranks of the Savaran cavalry and provide a loyal base to undermine the upper nobility and Magi, he standardized taxation, he centralized power in his own hands, and he abolished the threatening office of supreme general in favour of a more efficient quadripartite military-administrative division of empire.
He founded the Academy of Ghondishapur, and encouraged the intermingling and adoption of ideas from Rome, Greece, Armenia, Persia and India, whether philosophy, mathematics, science, medicine or literature. He accepted and embraced the Neo-Platonist philosophers who fled Justinian's empire, he was tolerant of other religions, he founded the first bimaristan hospitals, he defended the empire's borders with massive walls (the Derbent Wall, the Great Wall of Gorgan, the Wall of the Arabs and the Wall of Tammisha), and he initiated major building projects in the empire's interior, including a huge new palace at Ctesiphon and the Nahrawan Canal.
His alliance with the Western Gokturks overthrow the Hepthalites, who dominated Central Asia and had long embarrassed the Sasanian rulers with their demands of tribute and previous major victories. He then overthrew the Gokturks. He exploited alliance opportunities that Justinian failed to act upon, thereby seizing Yemen from the Axumites in support of an exiled prince and thus controlling part of the Red Sea trade. He fought with success against the Romans, negotiated favourable treaties with them, and in this context used propaganda better than most. When he successfully invaded Roman Syria, he relocated the people of Antioch to a new city called 'Khosrow's Better Antioch'! He entered Apamea in order to make Justinian's favourite chariot team lose, and he bathed in the Mediterranean.
