The group of elements that have the highest reactivity with water, high thermal and electrical conductivity, silver-like luster, are highly ductile, and have high melting points are the alkali metals. These elements include lithium, sodium, potassium, rubidium, cesium, and francium. Of these elements, sodium is the most reactive with water, has the highest thermal and electrical conductivity, and has the highest melting point. Therefore, sodium would be the element that best represents this group of elements.