Monsters, Inc. The animation was excellent for the time. The story, humor, and emotions were all excellent. I highly recommend it.

As for the isekai aspect, it was very creative. Allowing people to travel between the two worlds with doors in a children's movie setting really makes it stand apart from prior works with similar ideas, like Stargate (which allows travel with gates). I liked the idea that these were the monsters hiding in the closet growing up.

Wizard of Oz or Pan's Labyrinth perhaps