Anywhere from 1 to 5. 6 is just too thin-skinned/squishy/falling apart (still perfectly fine for including in a meal, just not as a snack on its own) and 7 obviously even more so.
A 1 banana and a 5 banana are basically completely different fruits. 1 is sharp and "grassy" while 5 is sweet and mild. So they're for different moods.