My personal favorite is :

This 404 page, of a Russian site is my favourite so far. It has a subtle parallax effect; with the stick figures dancing... to music! Yep, and it's not a single track on loop, there's a short playlist of groovy Russian(?) songs. A great find for a lazy Saturday afternoon. :)
I like Bitly's 404 page. :D Here's the GIF:

Nouveller has a brilliantly designed 404 page. It quickly presents you a terminal where you have to enter the magic word. ;)
This is all you'll ever need:
https://http.cat/