What is a highest oxide of element?
Highest oxide of an element is the oxide compound of the element which has highest possible value of oxidation state of the element to which the oxygen is bonded. Particularly, the highest oxide of period 3 is chlorine heptoxide, $\ce{Cl2O7}$ in which chlorine's oxidation state is +VII.

In general, the highest oxide of transition metal which has oxidation value of + VIII is ruthenium tetroxide, $\ce{RuO4}$ and osmium tetroxide, $\ce{OsO4}$.