Surely there wouldn't be just one. If you started with a list of common misconceptions, presumably a well-informed person would not have most of them.

e.g. http://en.wikipedia.org/wiki/List_of_common_misconceptions, which is quite interesting, though it's missing the first two I thought of: gods and the Monte Carlo fallacy.

A more interesting variant of the question might be, what do you believe that most well-informed people don't? That's harder.