Every king is more-or-less a conqueror. Either he wages war against his neighbouring kingdoms or defends his piece of land against foreign invasions. Now, who is evil & who is good depends upon perspective & is decided by the outcome of these wars or more precisely, whose media machine works better. Ravana had personal claims to be settled against Rama which led to the abduction of Sita & the war. But, note, Ravana inflicted no cruelty on Sita when she was captured. Also, he became Rama's priest during the Durga puja. He was a man of great knowledge & wisdom. Almost none of his actions were that evil. 
Hiranyakashpu was basically a non-Aryan king who was subdued & killed by the Aryan invasion. He & his brother were so powerful that Lord Vishnu had to take two incarnations to kill them. Thus Hiranyakashpu was a king wwho resisted the invading opposition. His cruelties were a result of the war. 
Now, coming to Kangsha, he was no doubt a powerful ruler. But these were his atrocities : 
- He was power-greedy, almost a dictator to the point that he destroyed the age-old Yadava culture of democracy (yes, many clans preferred “ganatantra” opposed to “rajtantra”)
- He imprisoned his father Ugrasena & subdued others who protested.
- He brought upon relentless oppression on his subjects - increasing taxes, sentencing those to death who protested, arson & murder.
- He was so frightened at the so-called prophecy that his own nephew will kill him, that he imprisoned his own sister & brother-in-law & killed their children one by one. In fact, this infanticide was the main reason people under his rule revolted & nobody stood by him when Krishna & Balaram killed him in open court.
- He left no stone unturned to find out the young krishna, by sending out assassins (Bakasura, kalianag, etc.) to search & kill the young child.

Hence, in my opinion, Kangsha is the most evil - the evil incarnate !!!
|