My favorite would probably be shitskin.
It's not very common outside some parts of the Internet, but it has the advantage of covering a lot of groups, and it's not only insulting, but also disgusting.
Imagine having someone calling you by the characteristic that your skin has the color of shit. If it sticks with you, you'll end up with a complex like Indians that pay a lot of money for those skin-whitening creams.
Your skin is shit. When people see you, they see shit. It's dirty, smelly, disgusting. That's what you are, a shitskin. Very dehumanizing, as marxists would say.
Other than that, I like nigger as a prefix/suffix. Curry nigger, niggerlover, sand nigger, double nigger, chinky nigger...