C'est pas le pogo le plus dégelé de la boite.

It kind of translate to: He/she is not the most unfrozen pogo in the box.

It implies that someone is not the smartest person around, actually the opposite.

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".

In Dutch: 'graftak' is a beautiful way to indicate an annoying/terrible person. Literally: 'grave-branch', which sounds nowhere near as satisfying.

Australia:

"Fuck off ya fuckin gronk!"

Or Ya fuckin mutt cunt!!"

"No me friegues, cara de tostada" don't fuck with me, tostada face. Much worse if the target is a spanish speaker.

It's not an insult but a way to show frustration without hurling insults.
So here in Russian like
ЁКЛМНЭ
ЁПРСТ
Ёшкин кот
Блин (pancake) to not use Блядь (whore)
Мать твою за ногу
Пиздец
Ппц
hmm that's all that comes to mind unless going into really insulting words.
In romanian I know only "Du-te in pula" "Du-te in pula mea" which is Get Fucked, fuck off.

Me cago en la leche de tu puta madre

That's one I picked up in Málaga. Fun place haha

Swedish Skitstövel - shit boot, basically means asshole

Despite being half Cypriot, I don't speak much Greek. Apologies to actual Greeks who realize how badly I'm butchering my transliterations here; my ancestors would be ashamed. (Most of them would probably just laugh at me though.)

That said, one of the funnier ones that used to go around in my family works like this: You yell for me from across the house. I respond "Di?" ("What?") And you yell again, usually meaning I should come over to wherever you are. Again, I say, "Di?" ("What?") And in exasperation, you respond, "Si kephali su tribidi!" (Your head is full of holes!) with emphasis on the last, matching, syllable as if to say "that's what!"