Which is correct?
A) Grammar is a subject of the upmost importance.
B) Grammar is a subject of the utmost importance.
Whether or not you agree with the statement, the correct answer is B. Many people mistakenly use “upmost” when “utmost” is the word they want. The confusion stems from not only their similar sounds but also their similar meanings. (And probably because “upmost” looks more like a real word.)
According to Merriam-Webster, “upmost” is the shortened form of “uppermost,” which, as you would expect, means “situated in the highest or most prominent position,” whereas “utmost” means “situated at the farthest or most distant point” or “of the greatest or highest degree, quantity, number, or amount.”
It helps to think of “uppermost” in a literal sense (e.g., on the highest shelf) and “utmost” more figuratively (e.g., of the highest importance): the reference books of utmost importance were located on the uppermost shelf. When in doubt, use “utmost” (how often do you come across “uppermost”?), or, better yet, substitute a word whose meaning you’re sure of.