Except, excerpt, exclude, excuse, (expedect), expected, (ex is a prefix), xylophone, and there are tons of "Q" words, so the answer is Z.
Humans are evolving they said. Here, on this thread, we're debating over the least used letter of the alphabet. Genius.
their reply to your comment still shows what you said silly goose #exposed To me, it's a close call between z and x but I got to say that z takes the cake.