  In the present paper, we describe how the band structure and the Fermi
surface of the iron-based superconductors vary as the Fe-As-Fe bond angle
changes. We discuss how these Fermi surface configurations affect the
superconductivity mediated by spin fluctuations, and show that in several
situations, frustration in the sign of the gap function arises due to the
repulsive pairing interactions that requires sign change of the order
parameter. Such a frustration can result in nodes or very small gaps, and
generally works destructively against superconductivity. Conversely, we propose
that the optimal condition for superconductivity is realized for the Fermi
surface configuration that gives the least frustration while maximizing the
Fermi surface multiplicity. This is realized when there are three hole Fermi
surfaces, where two of them have $d_{XZ/YZ}$ orbital character and one has
$d_{X^2-Y^2}$ {\it for all $k_z$} in the three dimensional Brillouin zone.
Looking at the band structures of various iron-based superconductors, the
occurrence of such a "sweet spot" situation is limited to a narrow window.
