Most characters in GOT are layered and complex, but not Ramsay Bolton. He is pure evil, cold-blooded, and vicious. Ramsay debuts in Season 3 and dies in Season 6.
Ramsay Bolton
One of the most detested characters in GOT was Joffrey, son of Cersei and Robert. He is a vicious and immature boy who abuses his power when he becomes the King of the Seven Kingdoms.
Joffrey Baratheon Lannister
Tywin Lannister is an old vicious man who ordered many of his soldiers to torture and killed the Targaryens, only to keep his own family in power.
Tywin Lannister
Viserys Targaryen sells his sister Daenerys to a warlord before threatening to cut an unborn child out of her stomach. He often threatens as well as violently puts his hands on Daenerys.
Viserys Targaryen
The most despicable characters in Westeros don't hold a candle to Walder Frey. The cowardly, treacherous, opportunistic lord of House Frey is best remembered for his role in "The Rains of Castamere".
Walder Frey
The High Sparrow's sadistic and controlling moral policing was despised by everyone. So much so, that at one point we all felt sympathetic towards Cersei Lannister.
The High Sparrow
Cersei is the Queen of the Seven Kingdoms. She is a cruel, domineering, selfish, and short-sighted woman who only cares about her children and brother/lover, Jaime.
Cersei Lannister