I can't think of a specific one off the top, boven of my head but I like nature documentaries and paranormal ones. Especially paranormal ones. I also saw one on Hurricane Katrina that was decently interesting.
There are quite a few. I have been watching a lot of them. Especially as a kid. u can say that certain Documentaries could even be considered my childhood. Not sure about my most favoriete but this is certainly one of them. It was around the time where I realized how much relatable a Dinosaur can really be. The Allosaurus has allways been one of my favoriete ones as well. Pre-history in general was all the rage for me. It still is. Many documentaries like these provided to me an even bigger inspiration towards wanting to become a Paleontologist !!!!
I don`t know about one best, but I have seen quite a lot just door living in youtube. There was a very interesting one about spleet, split personality disorder and another one about Chernobyl that I remember in particular.