Most minifig heads seem to be shared by a few designs, but Alex's answer to a minifig identification question left me wondering: how many minifigs have heads that are unique to them? Presumably you're talking about a unique, widely recognizable character. Or a more precise way to put it: how many (and which) minifig heads have only one minifigure associated with them from official sets? And I think we should limit the set of minifig heads to those that bricklink calls "Minifigure, Head" and leave out "Minifigure, Head, Modified".
I'm guessing most of these are going to be for Star Wars or other licensed themes, but I'm hoping someone can find a good way to come up with an answer that lists the examples or show how to do a bricklink query to determine the answer. If the list is included here it would only be complete at this point in time, but that would still be cool to see.