I'd place Constantine at #2. The rest is in good order. I find Hadrian much more interesting, but maybe switch Hadrian and Trajan too only because Hadrian pretty much continued Trajan's policies or made new policies in reaction to something Trajan did. (withdrawing from Mesopotamia for instance)

True, Trajan and Hadrian could possibly be changed around.  I guess I put Hadrian first because my field makes me more biased toward him.  Although Trajan defeated the Dacians and started the accumulation of their wealth, Hadrian really seems to have used the Dacian booty to flex Rome's prestige, possibly like no other time in its history.

I would rank Constantine lower because hs splitting the Empire actually weakened it.  He made an organizational change that should have been remedied by reform of the way in which provincial governors were chosen.  Instead he created two competing power blocks that fatally weakened the western empire because the Eastern Emperors then had no incentive to assist their brethren in the west when they were invaded by the Visigoths.  Instead, the easterners adopted a bunker mentality.

Scout – Diocletian was really the emperor who divided the empire, so perhaps you take more issue with him at #5 (I believe Constantine actually consolidated the divided empire after he defeated Licinius, which is why the former was able to have his pick of where he wanted to reside as emperor).You could well be right that Diocletian weakened the empire by dividing it, though I'm sure that this is a debate that historians could go on for a while about with competing arguments.  But it's a good and interesting debate (I think this is the closest thread we have on that topic).I realize that opinions naturally will differ about assessments regarding “significance” (half the fun of such lists) but I will say that I put Diocletian at #5 because I felt he saw the mounting threat to Rome's long term success and he did something about it, which changed the course of the empire (and world history).  One could also say that his harsh persecution of the Christians played a part in Christianity's early growth and as a kind of catalyst for monasticism.

I will add to this that Diocletian’s division of the empire into East and West actually bought it a few hundred years of extra time.  It wasn’t enough in the end, but it helped.  Just think that in the 50 years leading up to Diocletian, there was a string of 25 different emperors who reigned.  So in addition to internal strife, the Romans had to battle barbarians to the north and Persians to the east.  All of this on top of devalued currency….something had to be done.