There are actually various characters that a person could dislike, not the least for their apathy and blind faith in the face of mounting evidence to support the belief that the pigs had no genuine interest in the welfare of the animals in general.
Predictably, Napoleon and Squealer would have to be the two that are disliked the most - despised actually - due to their advantage over the animals which they abuse, mercilessly. With power comes responsibility or in this case - corruption.
After Old Major's visionary ideals, it is not difficult to manipulate the animals to make them feel as if questioning those ideals would be a betrayal of Old Major.
Propaganda is a despicable means of manipulation, appealing to emotions and not logic. It plays on fear, guilt and uses loaded language making it difficult for some to even understand what is being said. Squealer, the master of propaganda who can even turn black into white" is able to play on the weaknesses of the animals and feels no remorse. He has no conscience but does have an unwavering loyalty to Napoleon. His ability to use communication to his own ends, renders him a hateful character.
Napoleon does not have any redeeming features. He doesn't even start out with the same ideals - the ideals of Animalism and purely uses the opportunities presented to him to further himself. His interest in Animal Farm is only in the controlling of it, not its wellbeing. "Comrade Napoleon" is able to continue his rise to power because
at least no animal is lorded over by a human.
This seems to be the enduring characteristic throughout and , ironically, at the end, the pigs and humans are indistinguishable.
They no longer need to fear that Jones will come back to make their difficult lives miserable; he has already come back in the form of Napoleon and the pigs.
The tyranny is complete and no person (or animal) should ever be so lacking in compassion and so self-absorbed as to have no regrets at the lengths they will stoop to.
“ALL ANIMALS ARE EQUAL BUT SOME ANIMALS ARE MORE EQUAL THAN OTHERS.”
This sums up the story and shows the true nature of a person / animal who will misuse power to this extent and betray his fellow "comrades.".