The basic character of metal oxides decreases from left to right in a period due to decrease in electropositive character which in turn decreases the polarity of bond as well as the internuclear distance between the oxygen and metal atom. Therefore, alkali metal oxides are most basic and halogen oxide (oxygen halides) are most acidic $\therefore {\mathrm{K}}_2\mathrm{O}$is most basic metal oxide.