Since I only have one guess remaining, I'm choosing 'i' because it's one of the most frequent letters in English that I haven't guessed yet. The letter 'i' commonly appears in 6-letter words with 'e' in the third position, such as "client", "priest", or "viewed". Additionally, 'i' is a vowel, and the word already has one vowel ('e') confirmed, making it likely that another vowel appears in the word.