A melting point is the temperature at which a solid becomes liquid. It is determined by the thermal energy required to break the bonds that hold the atoms and molecules together.
A good solid should have a clear and well defined melting point. This is because the crystalline lattice structure of an ionic compound makes it easy to predict where and how much heat is needed to melt all of the component ions or atoms in a single unit cell.
Usually the best melting point is obtained by packing the ions of a given ionic substance well. This happens because the ionic bonds are relatively strong (remember that each cation or anion has one electron).
The most important factor in the successful packing is the radii of the ions, and the larger they are, the stronger the force.
This is especially true for the smallest ions. This is why barium oxide has a higher melting point than NaCl, as the barium has a significantly greater ionic diameter than the sodium cation.
Another ionic compound with a higher enthalpy of melting is magnesium oxide. It has a well-defined lattice structure, and it is known to have an impressively large number of atomic electrons (the molecular oxygen has about eight).
In general, the highest melting point of a solid will be the one that has the strongest ionic bonding (in the oh so clever coulombs law sense), as well as the most impressive intermolecular attractive forces. The most obvious example of this is the ionic bonding between fluorine and sodium in NaF.