It's archaic but I love "raclure de bidet". Comparing someone to the stuff you would scrape off of a bidet where all sorts of people have washed their taint. Short and loaded with contempt.

"Gibier de potence" is great too. Means "game for the gallow", with the term "game" using in the hunting sense. Basically someone you think should be executed.

Also "chien sale", dirty/unclean dog, which for a reason is a stand-in for "asshole".