
Ed M. answered 02/17/16
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I agree with Morgan R. that b) is "correct," but I really think a strong case could be made that a) is correct also, and indeed b) might even be said to be just a more emphatic variant of a). Notice that the only difference between a) and b) is the presence of other premodifyng car in the noun phrase any car that occurs in both a) and b). And no one could reasonably deny that any car in the showroom implies that there are certainly more than two cars there, which normally would require the superlative (i.e., most + adjective) when comparisons among the cars are made.
Yet using any changes the equation in that now the comparison is thought of as not one above the rest, but rather one-on-one or a sort of "round robin" comparison; in other words, you might be saying "Put this car up against each of the others (in turn) and it will be the more expensive one every time." This is why I believe that more expensive than any car is technically correct, despite what language or logic purists might maintain, and really the addition of other before car just emphasizes the comparison.