Have we been watching the same show?
Well, yes?
Please tell me what exactly do you dislike about her?
All right.
The main thing that makes Cersei so uniquely vile and despicable is not really what she does (she does horrible things, sure, but the same could be said of many characters, and she's not
quite the worst in that metric), but
why and
how she does it. She never does anything out altruistic concerns (her "concern" for her children is extremely possessive and therefore fundamentally selfish), or because she's advancing the greatness or anything she believes in, or out of pragmatic self-interest, or even because she's "just evil" like the two previously mentioned. Rather, she just gives in to all the darkest, most evil human instincts: spite, paranoia, pride, envy, cruelty, possessiveness. She's a slave to her emotions, and the emotions she usually feels are the ones a good person should
never give in to. This leads her not only to hurt people around her, including people who meant no harm to her whatsoever, but also, very often, to hurt herself. And since she's so prideful and unable to ever question herself, she ends up blaming others for her self-inflicted damage.
tl;dr she's a spiteful, paranoid, self-righteous sadist.