Gallienus, he led the Roman empire during the nadir of the crisis of the third century, and managed to prevent the total collapse of Roman civilization. While he failed to prevent the secession of the Gallic empire and the de facto independence of Palmyra, he defeated a number of Governors and commanders who attempted to secede or usurp the throne, while at the same time defeating multiple barbarian invasions. Even just a few more breakaway attempts succeeding could have caused a domino effect of secession which would have destroyed the empire.
He also heavily reformed the Roman military to handle barbarian invasions through the creation of mobile armies that could quickly be dispatched to deal with border incursions.
Without Gallienus the crisis would have been much worse, and the empire would have been less equipped to handle it. The empire could have fragmented into numerous successor states which would easily be overrun by invasions from outside powers. Aurelian reunited the empire, but he did it with Gallienus's military.
He also reigned for 15 years before getting murdered, which is extremely long by third century standards.
Gallienus is interesting for a similar reason to Maximinius Thrax: both emperors bookend the usurpation of the Senatorial elite by the professional soldiery. Gallienus was the emperor who opened the officer corps to the (shock horror) equites, including career soldiers.
Gordian is also interesting in the same way, a wizened old aristocrat dragged out of peaceful obscurity to lead a revolt against the unpopular barracks emperor.
