The current word pattern is '_____on_' (8 letters in total) and the letters we've already guessed are a, e, i, n, o, r, t. With 5 wrong guesses already made (out of 6), it's important to choose a letter that is statistically common and might complete a possible word. 'S' is one of the most frequent letters in English and is a good candidate, potentially forming a common ending when combined with 'on' (for instance, words may end with 'son'). Thus, I'll guess the letter 's'.