The English letter used least frequently in written English is Z.
Letter-frequency rankings come from counting letters across a chosen body of text, so exact percentages vary by corpus, dialect, genre, and whether proper names or technical vocabulary are included. Across many standard counts, however, Z sits at the bottom of the 26-letter alphabet, while J, Q, and X are also unusually rare.
A commonly cited analysis places Z at roughly 0.09% of English letters. Its rarity reflects the limited number of ordinary English words containing it, although loanwords and specialist terms can raise its share. American spelling also uses Z in forms such as “organize,” whereas many British spellings use S in “organise.”
Frequency is not the same as usefulness: Z can still be valuable in word games because it sharply narrows possible answers. The most frequent letter is usually E, but the exact leader and percentages depend on what is counted.