She's lovable. And people tend to like characters that moves them in a way. Cersei is far from perfect, and that makes her likeable. I srsly don't understand people whose fav character is Jon Snow, he's like the most boring ever.
I actually believe I don't have a favorite character, Jon Snow I liked at first, until he found the girl, that changed his personality a bit... I guess I would had loved to see Ned Stark on the throne... but he didn't last much did he?