A few other things that make a good wallpaper.
*You can look at it for more than a minute without getting tired of it.
*You can put a bunch of icons on it and that clutter does not have to compete with the clutter of a busy wallpaper.
It looks good.
I think numbers 5 and 20 would work the best, with 12 and 13 as runners-up.
I currently use ripple on XP. It is 800 x 600. The monitor is 1280 x 1024. The outside border is solid blue (R=O, G=64, B=152) but it looks like it varies slightly due to the inside picture. Autumn, Azul, Bliss, Crystal, Home, Peace, Radiance, and Wind are good and I have used a few of those.