What I love about these types of Japanese art is how the magic and the surreal become normalised in a way that western films rarely go anywhere near. I'm not a big anime fan by any stretch of the imagination, but films like Spirited Away and my personal favourite: Tekkonkinkreet just blow my mind with the sheer creative breadth of the vision.
Tekkonkinkreet is great! it's surreal and over the top but somehow still very grounded.
Some other awesome experimental animes that I think are worth watching are Redline, which has some amazing animation during car races on alien planets, and Dead Leaves, which is a classic escape from jail plot on steroids.
[1] https://www.youtube.com/watch?v=PfQjc2hs34Y