Molecular orbitals of heteronuclear diatomic molecules
The rule as generally stated in textbooks is that “atomic orbitals of the more electronegative element are lower in energy than the corresponding orbitals of the less electronegative element”.

Now, MO diagrams are only simple for elements of the second row of the periodic table ($\ce{Li}$ through $\ce{Ne}$). Involving heavier atoms makes it harder to guess at molecular orbital diagrams, and there is need for quantum chemistry calculations. Thus, the rule becomes: the further to the right your element is, the lower its energy levels are. The reason for this is that the charge of the nucleus increases with its atomic number, and thus lowers the energy levels.