Caligula deserves number one spot. Here is a summary of some of the many activities in his life as ruler of arguably the greatest empire in history:
He attempted to instate his favorite horse, Incitatus (“Galloper”), as a priest and consul, and ordered a beautiful marble stable built for him, complete with chairs and couches, on which Incitatus never sat.
Once, at the Circus Maximus, the games ran out of criminals, and the next event was the lions, his favorite. He ordered his Guards to drag the first five rows of spectators into the arena, which they did. These hundreds of people were all devoured for his amusement.
A citizen once insulted him to his face, in a fit of rage, and Caligula responded by having him tied down and beaten with heavy chains. He made this last for 3 months, having the man brought out from a dungeon and beaten, until Caligula, and the whole crowd that gathered, were too offended by the smell of the man’s gangrenous brain, whereupon he was beheaded.
Caligula’s favorite torture was sawing, which topped another list on this site. The sawblade filleted the spine and spinal cord, from crotch down to chest, and the victim was unable to pass out due to excess blood to the brain.
He also relished chewing up the testicles of victims, without biting them off, while they were restrained, upside down, before him.
He had another insulter, and his entire family, publicly executed, one after another, in front of a crowd. The man and wife were first, followed by the oldest child and so on. The crowd became outraged and began to disperse, but many stayed in morbid fascination. The last of the family was a 12 year old girl, who was sobbing hysterically at what she had been forced to watch. A member of the crowd shouted that she was exempt from execution as a virgin. Caligula smiled and ordered the executioner to rape her, then strangle her, which he did.
He publicly had sex with his three sisters at banquets and games, sometimes on the table, amid the food. He was finally murdered by the Praetorian Guard and some senators, leaving the Circus Maximus after the games. His body was left in the street to rot, and dogs finally ate it. He had ruled for 4 years.