There's a general consensus on which Roman emperors were "good" or "bad", but which emperors get too much credit, and which emperors don't get enough? I'll go ahead and get us started.
Most Overrated Emperor: Trajan
He obviously gets a lot of credit for being at the helm when the Empire reached it's peak size. But that came at the cost of stability in the East, which was thrown into chaos when all the legions were on campaign in Parthia. He also paid little mind to the succession, and if it weren't for the foresight and initiative of his widow, it's likely that civil war would have followed. In my opinion, he didn't seem to have any grand strategy for the empire, which isn't totally his fault, since the empire was at it's zenith, and there's no way he could have foreseen the problems to come. But in my opinion, it is worth mentioning because his successor, Hadrian, did appear to have a long-term grand strategy, despite also ruling during Rome's golden age. The glowing portrayals of his reign are certainly also due to the fact that a ton of major Roman historians lived during his reign (Tacitus, Suetonius, Pliny the Younger). To be clear, I don't think he was a bad emperor, and on a scale from Hadrian to Honorius, I'd probably put him just ahead of Claudius and behind Marcus Aurelius. I just think he gets a little too much credit for actions he took that were unsustainable, and the credit he gets lead to some people overlooking the potentially dangerous things he did.
Other honorable mentions, in my opinion:
--Vespasian: overrated because of his regime's domestic propaganda and sponsoring historians, also he overtly sold public offices to the highest bidder, which is pretty blatantly corrupt
--Julian: just opinion really, he seemed too wrapped up in personal stuff to be an effective administrator
--Antoninus Pius: kicked all the looming problems down the road for Marcus Aurelius to mop up
Most Underrated Emperor: Domitian
I'll first defend Domitian's mistrust and hatred for the senate, which is what got him assassinated to begin with. Some historians think this may have stemmed from a crucial part of his early life: Domitian was in Rome, and was only 18 years old during the Year of the Four Emperors. He was in the city when Galba was assassinated, when Otho committed suicide, and when Vitellius was dragged out into the forum and murdered, while the senate flipped sides with every victorious army marching in. This is a pretty plain demonstration that by this point in history the senate had no real loyalties except to whichever way the political wind was blowing, and it gave him a less naive perspective on the perils of his own office than Vespasian or Titus probably had. This is also probably why he went to such great lengths to endear himself to the Praetorian guard. But enough of that, on to why he should be viewed in a more positive light.
He was really the first emperor to view the empire as a body of provinces on more-or-less equal footing. He spent a lot of his reign traveling to administer the provinces, to the chagrin of the senate, which makes him the spiritual forbearer to Hadrian, in my opinion. Furthermore, by expanding the pool of civil servants to encompass men from the provinces, he was one of the emperors who made the ascension of men like Aurelian and Diocletian possible a century later. Not that he's singularly responsible for any overwhelmingly positive stroke for the empire, I just think he gets a bad rap when in actuality, he was the best Flavian emperor and probably in the top five best emperors to date (not that that was too special since there had only been 11 emperors). On a scale from Hadrian to Honorius, I'd put him ahead of Septimius Severus and just behind Theodosius the Great.
Other honorable mentions:
--Septimius Severus: made efforts to modernize the imperial military and address succession problems
--Aurelian: reunited the empire during the 3rd century, nuff said
--Tiberius: ran a tight ship, not much else to say, I just think he gets a bad rap because of his unruly subordinates
Keep in mind, this is all pretty much arbitrary and up to individual opinions, I'd just like to hear what everyone else thinks.
EDIT: If anyone wants to throw out some under/overrated Byzantine Emperors, I'm game.