This subreddit is full of insightful, fascinating and illumianting answers/posts by knowledgeable flaired redditors. Which of these would you strongly urge that everyone should read?
The next one.
There are so many good answers and well-researched responses that it's impossible to say. I'm always pleasantly surprised what I find and learn here, and I personally hope that I've contributed in some way.
Many of our flaired users maintain Profile Pages highlighting what they consider to be their best answers. Definitely a good place to start.
Anything that makes the Sunday: Day of Reflection weekly feature.
I don't have any specific answers, but "most essential" is a somewhat specific term and I'm going to ride it home.
If there's anything essential in the breadth of historical consideration, it's the means of understanding history. New information emerges all the time, placing holes in some theories and validating others, so one's historical knowledge is always, to some degree, perishable. As a result, the most essential skill is in the interpretation of information, especially interpreting that information in as "neutral" a way as possible.
As a result, the most essential posts on /r/AskHistorians to my mind will be those that deal with how modern people find historical materials, interpret those materials, and how they draw hypothesis and theories from those materials. In my opinion, what makes a historian is the skill they display in this task above all others -- which means that anyone with the proper approach and an honest intellect can be a historian, irrespective of formal education or the circumstances of their life. History is a form of science, after all, and factual information is the resource science thrives upon. If one is capable of and willing to find that factual information (or the informations that are theoretically equally factual) in context of history, then that's the requisite basis to partake of the broader, modern world of historical study.
Of course, what happens after that basis is established is responsible for the remarkable diversity in various studies of history, and where things naturally get much more complex. But again, "essential". And what is most essential is honest interpretation of the information at our disposal, with as little bias as is possible.
tl;dr do we have a post that boils down to "ditch the History Channel and passionately absorb information concerning your favourite eras of history from a diversity of sources"? Because that is the essential thing.
Sort /r/AskHistorians by "top" posts and that is the best place to start in assessing what the AskHistorians community considers most fascinating.