And before some wise guy points it out, yes, I know THAT is a time-honored way by which imperial dynasties ARE started 
Emperor Hadrian.
His father was a Spanish slave, but he still managed to become emperor. He presided over the Roman Empire at it’s peak, he had Hadrian’s Wall built. What a cool guy!
There is only one EMPEROR–Trajan!!! Ol’ MUT would wipe the floor with the likes of Caesar and Hadrian(for whom MUT held mostly disdain).
I don’t know where did you get this from. I think you are wrong. Emperor Hadrian was a relative of Emperor Trajan, his father P. Aelius Afer reached the rank of Praetor before his dead (it was more than impossible that a non citizen, specially an slave, was admitted to any goverment functions, at least at that time). Here is a link
Back to the Op, I will first tell you who is my favourite bad emperor.
The man is Emperor Publius Licinius Egnatius Gallienus
This man was the as bad as Commodus, Nero or Dominitian, but he was no coward. I specilly love his Portrait by the true master, Edward Gibbon:
“History of the Decline And Fall of The Roman Empire”, Volume I, Chapter X, Part IV.-
The funniest thing of all is despite his indolence, and the fact that he couldn’t give a damm about the empire, he survided no less than 18 competitors. The circunstances of his death are also noteworthy. He was informed that a rival was advancing to depose him. He put on his armour and marched at the head of his legions to fight his opponent. His nemesis took refuge in a city and Gallienus started the siege. He almost succeded, when a conspiracy was formed to kill him. In the middle of the night, the traitors informed him that the enemy had made a sally, without waiting for his troops or putting his armour, he mounted his horse and marched to the battle, in the confussion and darkness he was murdered by an unknown hand, not before he named a worthy man as his succesor. A very complex character to say the least.-
My fauvorites emperors, though, are those great men that lived in troublesome times and despite that, performed their duty to the end. As Gibbon said in his work they diserved to live in better times.
Decius, and Constantine XI are great examples of this. They reigned at times when the Roman Empire was almost extinguished, (in fact the latter was the Last Eastern Roman Emperors.
Decius was one of the illirians Emperors (others were Valerian, Probus, Diocletian, Aurelian or Gallienus) who “restored” the Roman Empire after the crisis that happened after the death of Septimius Severus (that almost dissolved the empire).
Decius was a good soldier but also a stateman that tried to revert the causes of the decline, (he was a one of the worst persecutors of Christianity).-
He died together with his son, a promising young man who was also emperor, fighting the barbarians that would ultimately destroy the Western Empire: The Goths.-
In that battle, at the beguinning of the action, his son was killed right in front of him. The death of an emperor of course plumbeted the moral of the troops to the ground, but Decius said, in order to revive it, “The death of a single soldier, is no great penalty for the Republic”.-
The battle, though, ended in a total defeat of the Roman Legions and in the death of Decius, and quoting Gibbon Again:
“History of the Decline And Fall of The Roman Empire”, Volume I, Chapter X, Part II.-
Constantine XI was a hero, the only soldier in an unwarlike city, he alone, according to Gibbon, was worthy of being called a Roman. My post is getting long. The only thing that I will add, is that in a nowin scenario he mantained the dignity of his office.
He didn’t survive the storming of his city, nor the fall of his empire
Being a nominal Christian, I first thought of Constantine and Justinian. But the guy who really interests me the most is Gaius Caesar Augustus Germanicus AKA Caligula.
Gotta love a man who alternately kills and sleeps with family members, makes his horse a Consul, and then has a bug-budget porno movie made about him by none other than Bob Guccione.
What can I say, the man had class. 
The reason why he’s not at the top of my list is because of the whole Chalcedonian nastiness. Now, being Orthodox, I of course think the Byzantines were right and the non-Chalcedonians were wrong, but I think that a lot of bloodshed and division could have been avoided if both sides would have sat down and reasoned things through reasonably like reasonable men.
On the other hand, he was almost overthrown by chariot racing fans. Those were some interesting times.
Regurgitated fish guts…Yummmy.
Actually, my favorite emperor is Vespasian because he built the Coleseum of Rome ( ok, the slaves did.) and he is featured in the Marcus Falco Mystery Novels.
I’m so shallow.
Luckily the circus performer he got to become his wife saved the day and reasoned with the crowd.
Charlemagne would kick all their asses(literally) An empire largely built by leading your own armies into battle is worth so much more. Plus an enlightened(for the day) leader who cared about education, morals and science, and played the politic game masterfully.
Oddly enough, the reason I don’t favor St. Constantine over St. Justinian is all that nasty murdering your own family business that finally prompted St. Helena to set her boy onto the straight and narrow in no uncertain terms. I do note, however, that in both cases, the Emperors are usually commemorated as a pair:
Ss. Constantine and Helena.
Ss. Justinian and Theodora.
Probably because the Church wanted us to remember just how very important the influence of these two women was in their lives.
St. Constantine led his armies into battle quite often. St. Justinian reconquered much of North Africa, Italy, and Iberia. Both of them cared about education, morals and science, and played the politic game masterfully. I do not recall ever hearing about any sort of “Code Charlemagne”, but the “Code Justinian” is a foundation for much of modern law, directly or indirectly.
Saint Empress Theodora was quite a remarkable woman, indeed.
I also like Claudius–but only as portrayed by Derek Jacobi.
The Grave of Norton I.
Ranchoth
(I believe he’s also the grand-Czar of the city of “enablers.” 
)
Charlemagne was both French and German (Karl) emperor. In Danish we have named a star constellation in his honour (“Karl’s Wagon” – part of the Bear), is that also so in English?
Trajan invaded and defeated the barbarian tribes in Dacia, present day Romania, and settled it with Roman veterans and settlers. I have read that the Romanian word for the Milky Way is “Trajan’s Road”, but Romanian guy I worked with had never heard about it. Anyone know the SD on this?
- Rune
In English it’s called the Big Dipper. Dipper meaning something like a ladle used to “dip” in water to drink out of. There is also a little dipper among Ursa Minor(the little bear.)
Word of the day “Asterism”. Both dippers are asterisms, meaning a distinctive group of stars that is less than a full constellation.
Oh, poop. 
–SSgtBaloo
Let’s not forget John Tzimiskes and Basil II.
I’d also mention Heraclius, but I don’t feel he was great. A great strategist, yes, but not a great Emperor overall.
Let that be a lesson to all rulers – DON’T forget Secretary’s Day!