  Using the Eliashberg strong coupling theory with vertex correction, we
calculate maps of transition temperatures (T$_{c}$) of electron-phonon
superconductors in full parameter space. The maximums of transition
temperatures for superconductors are predicted based on the maps and the
criterion of instability of superconductivity. The strong vertex correction and
high transition temperature are tightly correlated in superconductors. We
predict that the maximum of T$_{c}$ of new iron-based superconductors will be
close to 90(K).
