What are your favorite (Youtube) video's/documentaries of all time that you can watch again and again?

Defunctland's Disney's Fastpass: A Complicated History is a fantastic dive into the logistics of managing a giant crowd over several years, from approaching it as an engineering problem all the way to exploiting the system for profit. It's such a fascinating problem to me, and it's presented in an engaging and fun way. The rest of the channel is fantastic too!

I'll also recommend Fascinating Horror - these aren't fictional horror stories, these are real disasters told in an incredibly detailed but highly respectful way. They don't just explain the disaster and why/how it happened, but also the regulations that came afterwards to make sure these would never happen again. They're all high quality, but if you want a starting point the Nutty Butty Caves will give you a good idea what the channel is about.