I've recomended this before to the same question r/askhistorians contributor and Cambodia historian ShadowsofUtopia has an amazing and insanely in depth series called In the Shadows of Utopia on the history of the Khmer Rouge. And when I say insanely in depth I mean it. The first episode is the standard Khmer Rouge story we've all heard the emptying of Phnom Penh, the discovery of the horrors of the regime. But the subsequent episodes are where the series really shines. He talks about ancient Khmer kings, the development of Marxism. The history of Cambodia-Vietnam relations. All of this building up the context and background of what happened along with the quirks and anxieties of Cambodian society, so we can understand how something as insane as Democratic Kampuchea could come to pass.

I've read several books on the Khmer Rouge and watched some documentaries and never quite found what I was looking for. Most media regarding them take a much narrower view explaining what happened and maybe a little bit before the revolution. This series does it's best to explain how and why this could happen, exploring topics as varied as Leninist theory and Pol Pot’s love life. It's long and currently in progress, but the episodes are engaging well thought out and really take the time to fully understand each topic. Plus, it just flows well, I can't recommend it highly enough

The problem I have encountered with my, admittedly limited, experience of YouTube historians is that YouTube videos need to be entertaining to be successful, and to be entertaining often comes at the expense of being transparent about one's sources. This doesn't mean that the research any particular YouTuber has done is necessarily unreliable or flawed, but it does mean that the audience essentially has to take their word for what they are discussing.

For example, this video by Invicta discussing the Nubian Archers mentions Herodotus' description of the Aithiopians in battle. The description in question comes from Hdt. 7.69, but we would not know that from the video in question. Rather than have a quotation of the relevant passage appear on the screen with a reference, the video instead opts for very well done art depicting an interpretation of an Aithiopian warrior. There is nothing wrong with this, documentaries have done the same thing for decades, having experts appear as irrefutable and reliable testimonies for events, but, as I said, it makes such videos inherently somewhat unreliable. Another example would be Historia Civilis' video on the Spartan Constitution, which attributes to Xenophon "the best surviving account of the Spartan way of life", without considering the fact that Xenophon's Sparta was a snapshot of fourth century Sparta, nor the inherent issues with studying the 'Spartan constiutution' (see my answer here). They also include a passage, like I criticised Invicta for not doing, but do not provide a detailed reference, only offering the work, not where in the work the quote is from.

This is largely down to the problems associated with visual media. Visual media is meant to be watched, and if you simply have a wall of text upon the screen rather than images or videos, then you are subjecting an audience to a medium they were not there for in the first place. Thus, YouTube historians are always going to favour watchability over watertight reliability, otherwise they'll lose their viewership.

Moreover, at the risk of sounding like a gatekeeper, YouTube historians are unlikely to be fully trained historians, or they will include people trained in necessary elements of video production but with no history training. This means that, no matter how well reseached a video, the development process is one of repeated iterations involving people who might know how to make a good video, but not how to offer 'good' history (this ties into my point above). u/JoshoBrouwers actually mentions this in his Ancient World Magazine post about a video he made contributed to for Invicta concerning Mycenaean chariots.

Then one must also consider the amount of research that goes into each topic. From what I have seen, while topics are always rather small in scope, there are only ever around five or so bibliographic references in the video's bibliography. Of course, such videos should be treated like introductions to topics, not definitive accounts, and the resources they provide should be treated as further reading. However, it would be bad practice to submit an academic article with so few references as there is always more to be read about any topic.

So far, I haven't really answered your question directly, and I am afraid I cannot comment on who are the 'best' history YouTubers as there are so many criteria by which to judge such a contest, such as what you want to get out of their videos. That said, I would recommend keeping an eye out for YouTubers who provide relevant resources, whether that be in their videos, the description, or in some form of companion material. Some YouTubers who do provide such references are Invicta (who also occassionally has academics contributing to research) and Historia Civilis, but they are far from perfect, as I have mentioned. I hope this helps your search.