The list is screwy — Diocletian was definitely one of the best emperors the Empire ever had, apparently he made this list because he persecuted Christians; he didn’t put the empire back together (that was the role by my personal favorite, Aurelian, who ruled for but five years, then was murdered by someone on his staff who was an embezzler), but he did split the empire into two halves, set up an orderly system of succession — there had never been one — built a retirement villa which still stands (at Split, in Croatia), retired, compelled his eastern empire colleague to do the same, and watched as their designated successors actually succeeded them both.
Tiberius also doesn’t belong on this list, and I noticed that the plot against him by his former daughter-in-law (and grand-niece) and Lucius Sejanus isn’t mentioned, even though Sejanus himself is. Tiberius was a delegator, as were all the better emperors, but he took power when he was already fairly old (his adopted father and predecessor Augustus ruled for over 40 years), so it’s not surprising he wouldn’t live in noisy, smelly old Rome if he didn’t have to — not least because he moved to Capri. Tiberius and his brother Drusus were probably the best brothers and generals Rome ever produced.
And any list of worst emperors that doesn’t put Elagabalus at the number one spot was clearly compiled by someone with their heads up their keisters (this doesn’t take into account the short-lived successors who bridged the gap between Nero and Vespasian, they weren’t up to the task, but didn’t rule long enough to give them a fair rating).
http://madmonarchs.guusbeltman.nl/madmonarchs/elagabalus/elagabalus_bio.htm