Sounds cliched but it has to be nigger. It’s like the holy grail of racial slurs and it makes some of them go ape. Over a word that literally means “black”
I also like Kraut, Paddy and Gweilo. Sheep shagger is another one I like for welsh and kiwis and aussies depending where you’re from. Sammy was used against hindis by the British which sounds funny.