That is a tough one, Aphrodite.

Constantine II ruled for seven years and seems to have been relatively ineffectual. Constans reigned for longer, but only ruled (as something approaching an adult) for a short time, and succeeded in alienating his troops. So I guess that the most "underrated" is Constantius II, because historians tend to dismiss him, however, he did succeed in holding the Persians off (through a variety of means, including bribery) and he did succeed in surviving the blood bath which swept away his brothers and the rest of Constantine's line. On the other hand, he had a goodly part in it.

I think that another reason he tended to be dismissed as 'ineffectual' is because he had decidedly Arian sympathies. Of course, if he had managed to produce a viable heir, he might have stabilized the imperial succession. Instead he paved the way for the tantalizingly brief and ultimately fruitless reign of his cousin, Julian the Apostate.

Regards,

Fabius