I used a 16:1 for a while on my 18sq.
It was basically a 8:1 cascaded with a 2:1 preload line.
This seems to be the most popular because it is cheap and easy to add to a boat which has an 8:1 and can be taken off easily.
It was overkill for the 18sq mast, so I switched to a split 8:1 to take the wrinkles out of the bottom of the sail.