Different materials have different properties. None of them is the best in all aspects.
When considering impact attenuation, the best are loose fills, such as sand and gravel. For outdoor installs, many seem to prefer artificial grass and rubber granulates. Natural grass is the only one which doesn’t grow hot in sunshine.
All materials have their weaknesses, as well as their strengths. Sand wears away at floor surfaces and climbing elements. Artificial grass and rubber granulates are rather expensive options. Natural grass – even when fully grown – wears easily and should only be installed on playgrounds with relatively little use.