I am partial to some of the Mexican insults we pick up here in the SW US, including calling someone a pendejo or conjo, or a cabron. (Respectively, a pubic hair, a female pudendum, and a goat). They are also liberal with smirches on one's immediate ancestry, as in hijo de puta (son of a bitch, or of a whore) or maricón - homosexual, a particularly strong insult in the land of macho.

A few other choice expressions:

Tu eres más feo que el culo de un mono - You are uglier than the butt of a monkey
Tu hermano no tiene la ingle - Your brother has no groin
Tu madre es muy gato y feo - Your mother is fat and ugly.