Constantius is certainly underrated, mainly because Ammianus Marcellinus painted him as worse than Nero, Caligula and Domitian. However, it is interesting that within Ammianus if you ignore the character assassination you detect a very able emperor who handled the succession from his father efficiently (if cruelly), kept the Persians at bay and dealt with the barbarians. Ammianus neither dissembles nor cloaks these successes, he simply weighs them down with damning narrative.

SWe have a glimpse of his contemporary reputation as a soldier, which was impressive and can be seen from the fact that so few cities endorsed Julian, fully expecting him to be thrashed by the Augustus. So I would agree, Contantius was the most underrated of the set.