I'm choosing "s" because:
1. It's one of the most common consonants in English
2. It could appear at the beginning of the word (as in "banana" but with an 's' instead)
3. It could also appear at the end of the word
4. Many 7-letter words with multiple "a"s might have an "s" (like "sagas", "savant", etc.)
5. Common letters like "s" give us more information early in the game