Water Vapor - (least dense)
Ice
Liquid Water - (most dense)
Explanation:
While you would expect that the density of a substance increases with lowered temperatures - because the particles get closer together hence there are more particles of the substance per unit volume at lower temperatures-, there is an exception in water. It is even called water anomaly.
Ice, which is the solid and most compact form of water, is less dense than water at 4° Centigrade. This is why ice sheets float on oceans in the polar regions.
