Another idea is a perfectly still pond, on a perfectly still day, and something dropping down from dead vertical into the centre of the pond, then the "wave rings" around the item should be theoretically perfect right? but then I guess the item dropping into the pond would also have to be...
What about bubbles in liquids? aren't they "trying" to be a perfect circle / sphere, I guess gravity could also be squashing them slightly out of shape.
Oil droplets in water? on the surface of water?