A lot to choose between, but my pick would be Theodosius I. Lived 347-395, so he had decades of possible emperorship ahead.
During Theodosius I time the Roman Empire could have incorporated the barbarian tribes into Rome, transforming the concept of "Roman" to a more modern, reality-focused meaning. After Theodosius death the roman elite closed ranks and kept "barbarians" such as Stilicho away from power (while they a century before had welcomed the Illyrian generals with Dominitian as the apex), thereby depriving Rome of good potential emperors and instead creating a lot of enemies.
Theodosius had married a niece to Stilicho, ruled (after 392) both the East and the Western empire and was a good general himself. Give Rome 20 years of stability under Theodocius, during which selected barbarians are co-opted into supporting Rome, reinforcing the military, blocking hostile barbarians and maybe have enough time to do some serious restructuring of the civil service (corrupt), tax system (far more corrupt and giving the landowning aristocracy huge advantages) and monetary system (stopping the rampaging inflation that made "money" farily useless).
Even none of above, just 20 years without successor wars, opportunistic invasions after the death of an emperor and chronic backstabbing among the court officials / military leadership would leave Rome far stronger than IOTL. Theodosius I:s sons were 18 and 11 years old when they succeeded him in East and West - and quickly became weak puppets. Let them spend 20 years gaining experience (or give Theodosius time to find good replacements) and the roman decline could be prevented another generation.