Are there other versions of the word foremost, like there are other versions of, friendly (friendlier and friendliest)?
Oxford Dictionary defines "foremost" as "most prominent in rank, importance, or position." An example in a sentence would be "Doctor Philips was the foremost in his field." There aren't any other forms of this word. Just try saying it; doesn't "foremostly" or "foremostest" sound awfully weird? Plus, in the example of "foremostest," it's redundant because "foremost" means the best, the highest, in that field. You can't get any better than foremost. So not only is foremostest grammatically incorrect, but it's redundant.