It is interesting where one can end up while surfing the internet. I don't even remember how, but a few days ago I happened on two links (one leading to the other) that discussed what fallacy or fallacious argument would be the worst.
The Australian professor James Franklin discussed the suggestion by his compatriot philosopher David Stove that the idea behind cultural relativism and postmodernism is the worst ever: We cannot know things as they really are because we only have subjective knowledge. His criteria were how bad he considered the argument to be and how widespread it was.
The Less Wrong forum member "Yvain" presented as their own choice what they called the "noncentral fallacy". It means to fallaciously invoke the emotional reaction that is appropriate for a typical (central) member of a category when faced with an atypical (noncentral) member. Yvain's first example is people making the argument that Martin Luther King was a bad person because he was a criminal, implying that his having been punished for nonviolent resistance is morally comparable to having committed burglary.
I have given the issue some thought and have, for the time being, settled on a different one. From my perspective, the Just World Fallacy is perhaps the worst.
Admittedly, nit-pickers might argue that it is more of a cognitive bias than a formal logical fallacy, but I don't want to pick nits here, and the two categories grade into each other anyway.
Anyway, for me it tops the list for three reasons: it is extremely widespread, it causes a lot of damage, and it is actually pretty obviously wrong once given a bit of thought, meaning that we could easily do better if they would only use our brains.
In case it is not clear, the JWF is the assumption that everybody more or less has and gets what they deserve. It is clear why we humans tend to believe this; many of us just cannot stand the idea that the world can be extremely unfair even if that is demonstrably the case. For one thing, it distresses most of us to see bad people get away with what they do, and to see others suffering undeservedly, simply because we have empathy. For another, the idea that we could be hit by a catastrophe ourselves also distresses us, and thus believing that nothing bad can happen to us if we follow "the rules" (be it morality or lifestyle choices) gives us a feeling of control.
In other words, the JWF is a form of wishful thinking, helping us to cope with reality by ignoring some of its less pleasant aspects. The problem is, it does indeed form the basis of many harmful beliefs and practices.
Perhaps most present to the average skeptic is its role in quackery. Many a proponent of "alternative medicine" believes that one can stay healthy (and, for example, will never get cancer) if only one follows a certain dietary regimen or thinks the right thoughts. In reality, there is a strong stochastic element to diseases, and especially to cancer, so it can hit anybody. But this prospect is apparently hard to cope with for some of us, much easier to convince oneself that some dietary guru's teachings or positive thinking give us control. The unpleasant flip-side of this is victim-blaming: under this logic, anybody who actually gets cancer could have easily avoided it if they had only followed the same teachings.
Speaking of victim blaming, that is another area where the JWF plays an important role. The idea that women will only ever get raped if they behave in certain ways - wearing revealing clothing, getting drunk at a party, walking home alone through a dark park, "asking for it", etc., is as widespread as it is ridiculously and demonstrably idiotic. And thus, of course, it causes tremendous harm as victims are vilified instead of supported.
The JWF also forms an indispensable aspect of libertarianism, neoliberalism, various brands of conservatism, and free market ideology in general. If the ideologue is convinced that our current society is already a sufficiently Real Free Market (TM) and a meritocracy, they will argue that everybody earns and owns precisely as much money as they deserve. If you were any good, somebody would be willing to pay you a high salary or you would have founded a successful business! If they believe that our current society is still too socialist, they will argue that we could achieve the happy state of a Just World if only we would privatise and deregulate everything. Inheritance, pure luck, discrimination, the fact that only a limited number of people could be billionaires anyway, and other such trifles are swept under the rug, and thus the most abject poverty and the most bizarre excesses of wealth accumulation are justified as, well, just.
Of course, more generally speaking the Just World Fallacy is the friend of any existing order. And that is just the problem. Once we fall prey to this cognitive bias, we become less motivated to create a world that would actually deserve that adjective.