There's no objective metric for "most disliked" emperor (we don't really have any approval rating polls to look at) but I'd say that Didius Julianus was arguably the most disliked emperor.

Didius Julianus became emperor after buying the throne from the Praetorian Guard. The Praetorians murdered Pertinax in 193, and not having a replacement lined-up, they decided to instead auction off the throne to the highest bidder. Didius ended up winning, and his "prize" was the imperial throne. The Senate begrudgingly named him Augustus under threat of the guard.

So Didius was now, at least nominally, emperor. Unfortunately, everyone realized how he acquired the throne and were none too pleased with his openly corrupt ascension to the purple. In public he was the subject of mockery and ridicule, and mobs of people would gather to throw rocks and berate their new "emperor". What's worst, many of the generals in the provinces completely refused to recognize him as emperor. Eventually one of those generals, Septimius Severus, marched on Rome to depose Didius. The Praetorian Guard, seeing which way the wind was blowing and realizing that Didius had virtually no supporters in Rome, declared their support for Severus. In turn, the Senate declared Severus emperor and ordered the execution of Didius Julianus. His entire reign lasted only a couple of weeks.

As /u/cjt09 already wrote, there is no objective way to determine the "most disliked". You can't put a number on that and compare different emperors. This is further complicated by the fact that the Roman Empire had a pretty diverse population. Different groups of people would have widely diverging oppinions on one and the same emperor.

The death of emperor Nero might serve as an example. You mentioned him yourself as a contender for most disliked ruler. And fittingly Sueton tells us in Nero's biography that supposedly people were celebrating when they got the news of the emperor's death (Suet. Nero 57). But then he goes on to mention others that decorated Nero's grave regularly with flowers and wanted to put up statues of him at the forum. It acctually seems as if Nero was quite popular with large parts of the Roman population.

The problem with assessing the general popularity of individual emperors is that the sources that tell us about them are pretty much all written by a very small group, the senators of Rome. And they were obviously biased in their judgement. Before the establishment of the empire they were the ruling class and the memory of that time was still strong with them. Emperors who publicly downplayed their own elevated status and showed themselves to be first among equals, like Augustus or Trajan did, would get a good press. Those that ruled in a more obviously autocratic manner, like Caligula or Domitian, would be described as "bad emperors".

Modern historians try to question this senatorial narrative. Even notorious cases like Caligula or Heliogabalus are now beeing reevaluated. You have to judge for yourself if this is always successful but never just take the negative verdict of ancient historiography at face value.

Although /u/ctj09 builds a strong case for Didius Julianus, I would like to propose Domitianus for the position - although the title is certainly shared by many. Domitianus was the son of Vespasianus and brother of Titus, both of whom co-reigned during the completion of the Colosseum (Vespasianus died slightly before), and he is one of the few emperors that we know recieved the treatment of damnatio memoriae after his death. That being the erasing of all memory of him.

Suetonius tells us this about his death:

The people shewed little concern at his death, but the soldiers were roused by it to great indignation, and immediately endeavoured to have him ranked among the gods. They were also ready to avenge his loss, if there had been any to take the lead. However, they soon after effected it, by resolutely demanding the punishment of all those who had been concerned in his assassination. On the other hand, the senate was so overjoyed, that they met in all haste, and in a full assembly reviled his memory in the most bitter terms; ordering ladders to be brought in, and his shields and images to be pulled down before their eyes, and dashed in pieces upon the floor of the senate-house; passing at the same time a decree to obliterate his titles every where, and abolish all memory of him. [---] They say likewise that Domitian dreamed that a golden hump grew out of the back of his neck, which he considered as a certain sign of happy days for the empire after him. Such an auspicious change indeed shortly afterwards took place, through the justice and moderation of the succeeding emperors. (Suet. Dom. 23)

Have a look at Elagabalus (reigned 218-222 AD).

Amongst the most hated because he affirmed some Roman suspicions about "eastern" effeminacy and luridness.

Reasons to hate him:

1. Only 14(edit) when he was emperor due to his grandmother bribing and assassinating for him.
2. Enjoyed "pranks" such as releasing animals like poisonous snakes, leopards, and lions into crowds or into banquets he lured poor people to.
3. Violated Roman religion and mores with acts such as violating and marrying a vestal virgin and removing Jupiter as the head deity
4. Appointed himself high priest of the new religion that he sort of made up as he went along (Elagabal the sun god was a preexisting deity, just not a well-known one)
5. Referred to the senators as "slaves who wore togas"
6. Divorced (threw out of the palace) his first wife on discovering she had a small birthmark that displeased him (the toe...Constanza more like "Can't stand ya"), all the while cross dressing and contemplating an ancient sex change (this is debated and the subject to much scholarly debate, remember that Romans had very different attitudes to sexuality than the Greeks or Persians).
7. Did other sadistic pranks like "making it rain" gold coins from a tall tower to watch as common folk crushed each other attempting to retrieve it.
8. Ridiculous expenditures like urinals made of onyx and naval battles staged in wine.
9. He once caged a monkey, lion, and snake together and tossed the genitals of his victims to them. Why? Cause Elagabalus, that's why.
10. Overtly ostentatious dress, including a tiara, hairnet, and a lot of perfume.

He died at 18, cowering in a latrine.

See Dio and Herodian for Primary sources. Also, see the Historia Augusta. Probably Sir Ronald Syme is the best guide to the H.A.

For newer books see Icks, Martijn Images of Elagabalus. (2008) and you can always check out the amazing, sensational prose of Edward Gibbon.