I found a useful (though simplified) scheme illustrating the energies of atomic orbitals, in crescent order:
I wonder if there was an analogous (simplified) scheme or rule for molecular orbitals, at least for diatomic homonuclear molecules, for which the electronic state is given by the so-called molecular term symbol
$$\displaystyle {}^{2S+1}\!\Lambda _{\Omega ,(g/u)}^{(+/-)}$$
where
${\displaystyle S}$ is the total spin quantum number
${\displaystyle \Lambda }$ is the projection of the orbital angular momentum along the internuclear axis
${\displaystyle \Omega }$ is the projection of the total angular momentum along the internuclear axis
${\displaystyle g/u}$ indicates the symmetry or parity with respect to inversion through a centre of symmetry
${\displaystyle +/-}$ is the reflection symmetry along an arbitrary plane containing the internuclear axis
Such scheme or rule would justify for instance the following succession:
$$\sigma_g(1s)<\sigma_u(1s)<\sigma_g(2s)<\sigma_u(2s)<\pi_u(2p) <\sigma_g(2p)<\dots$$
