It's a tie between "Because you're an insane degenerate animal and you deserve to die" from Breaking Bad and "Go lick a dog's ass 'till it bleeds" from Count Zero. I also have great affection for the South Park movie's "uncle fucker".
For actual use I tend to favor fuckwit, fucktard, dipshit and jackass. When I'm angry with someone I don't want to amuse them with a clever quotation.