Mercury and Mars don't have strong atmospheres, so rain on them is not possible. Venus - sulfuric acid. It is caused by the reaction of sulphur dioxide and water in the planet's atmosphere. Earth - water. Because of the water cycle. Jupiter, Saturn, Uranus, Neptune - diamond. Lightning in them could split carbon atoms from atmospheric methane, forming soot, which would then create diamond under combinations of high pressure, heat, then rapidly cool.
