Diocletian was a proto-feudalist monarch who disregarded the republican traditions of the Empire in exchange for Grek and Prsian traditions of divine kingship, quite literally demanding to be worshipped as a living god over all other men. He wrecked the already-severely-damaged economy, split the Empire into four pieces, enslaved the soldiers as conscripts serving life-sentences in the army, instituted a form of serfdom for poor farmers, bound children to the profession of their parents, embarked on a massive campaign of persecution of Christians because they hurt his precious godlike fee-fees, and was generally a twat. He is remembered fondly by some because he came in at the end of the 'Crisis of the Third Century', and was not wracked by constant civil wars until near the end of his reign.
Constantine I killed his eldest son and boiled his wife alive like a lobster, and then split the Empire like personal property between his much less competent younger sons, who all started fighting each other (thus killing innumerable Roman soldiers) as soon as Constantine died. He's remembered as 'the Great' because the Church slobbered all over his boots for making Christianity the de facto unofficial religion of the Roman Empire, and persecuting pagans.
Theodosius is only remembered as 'the Great' because the Church slobbered all over his boots for making Christianity the officially enforced religion of the Empire, instead of just the unofficially enforced one, and persecuting pagans (see a pattern?). Fuck him.
For some revisionist fucks:
NO, Domitian was NOT just 'slandered by those mean ol' Senators', gtfo. He was a paranoid, vain, autocratic tyrant who probably murdered his much nicer and more competent brother, Titus. The Senate and Praetorian Guard preferred Nerva, a 63 year-old man with no major achievements to his name, to the young and active Domitian, because his activity was overwhelmingly tyrannical.
NO, Gaius Caligula was NOT just 'slandered by those mean ol' Senators', gtfo. He was literally brain-damaged by a fever early in his reign and descended into arbitrary excesses because of it. He was so unpopular that the Praetorian Guard, Senate, and people of Rome ALL preferred his uncle, Claudius, who was thought (at the time) to be a chinless, crippled, stuttering, drooling, browbeaten, abused-by-his-wives moron with no real political or military experience. THAT'S how bad Caligula was.
Uncle Claudius WAS chinless, crippled, stuttering, drooling, browbeaten, and abused by his wives (all seen as very negative qualities by the Romans, who preferred physically flawless men with great gravity in their bearing and control over their household), with no real political or military experience, but he was also a positively brilliant man with a strong sense of justice and foresight in everything except his taste in women, and only PRETENDING to be a moron to avoid getting iced by Caligula.