This is all you'll ever need:

https://http.cat/

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. :)

Click on the image, and start dancing!

CSS Trick's 404 page completely suits the website. Here's the snapshot.

I like Bitly's 404 page. :D Here's the GIF:

This 404 page is very creepy. :O

This is made using HTML5 Canvas.

Github has smartly used parallax effect on its 404 page. Try to move your mouse on the image area.

Nouveller has a brilliantly designed 404 page. It quickly presents you a terminal where you have to enter the magic word. ;)