Watership Down.

The oblongs, ripping friends, Ren n Stimpy, I r baboon were some of the best. Drawn together was a gem too that came out when i was a teen

Vacalactica: https://www.youtube.com/watch?v=lF7tKWzkzi4

Grave of fireflies

I was 12 and just getting into Anime, fresh off seeing Akira and Ninja scroll and hungry for more, I was not ready for Wicked City.

https://en.wikipedia.org/wiki/Wicked_City_(1987_film)

Mid-90s, I used to stay up all night long on Fridays, watching weird cable access shows and infomercials. There was a Highlander: The Animated Series cartoon that came on around 4AM. No one ever believed me when I tried to describe it.

“The great mouse detective” had a fight scene at the end that freaked me out as a kid.

Bible Black.

Invader Zim

Danger Mouse - I think the consistent 4th wall breaking, kidnapping narrators, and sense of humour as a whole had an effect on my from a formative age.

Monkey Dust - the cartoon that made it clear to me that cartoons weren't not at all nesseccarily safe for kids. I was too young to appreciate it at the time, it was too disturbing for tween me.

Sealab 2021 and Excel Saga both crazy animations that I found easier to digest about that time, too.

Watership Down, other folks have already mentioned.

I think for me it was Robot Chicken that would play late at night

Bible Black and Gantz

If we're counting highschool as still a kid, the old Chinese cartoon Calabash Brothers. Still looking for ENG subs for the newer version that released in the 2010s, though.

It's a cartoon where a snake and scorpion demon escape from a mountain and an elderly man has to grow a magic calabash seed that turns into 7 different calabashes that turn into little dieties when ripe in order to stop the demons.

It’s a very generational question. Like, many of the cartoons I watched as a kid would definitely be considered crazy today (and racist, misogynistic, murderous, etc.) but at the time were typical.

Well there's many mentions so I'll add one I have yet to see listed, The Adventures of Mark Twain is always an unsettling classic.

Gantz, Watership Down, Animal Farm, Super Jail, and Urslua looks like my father's mother, whom I hated, so I freaked the fuck out when my mom brought that VHS he.

Muran Buschstansangur