Sherry over at Everything Language and Grammar had a post way back in May about some athlete who committed a venial grammar sin. Specifically, the athlete said that he had made “the stupidest mistake.” (Having made a number of gallingly stupid mistakes in my life, I can only imagine the depth of idiocy to which the stupidest mistake would descend.) Sherry claims that the athlete has made a further mistake by describing the mistake as stupidest rather than most stupid.
However, there’s no reason to suspect that stupidest is any less proper of a formation than most stupid. Personally, I prefer the former. I did a Google Books search (full view only, so as to eliminate the modern journals that were sneaking into the results) comparing use of the two possibilities before 1900 (at which point the presumed stupiding of the language may have been underway). 463 for most stupid, 407 for stupidest. Similarly, 820 for more stupid, 674 for stupider. So if stupidest has ceased to be a word, it must have done so some time in the last century. And that seems unlikely, given the number of modern people who have no problem with the word.