I’d easily have to say Ethnicity, Religion and Race. Blacks, Romani and Jews have faced genocides, slavery and persecution because of these reasons.
Social Class is probably a close contender too, as Wealth divided people a lot, especially in Europe, however Social issues have gotten extremely better in recent times.
Nationality is on par with with the top 3.
Politics don’t normally make a person hated, unless they are extremely Far-Left or Far-Right, such as Fascist or Communist, however in these cases they deserve it.
Gays and Biesexuals don’t usually get hated on, unless they make it really obvious by making a high pitch voice.
Genders don’t normally get much hate, unless unless someone claims they’re a third gender.
Overall I’d say Ethnicity as normally it is a summary of someones background.