When it come to my favorite summons, these are my favorites from games:
III: Don't one since I did not get far enough to obtain them
IV: Shiva, Asura, Leviathan, Bahamut
V: Shiva, Carbuncle, Phoenix, Bahamut
VI: Bahamut, Shiva, Gilgamesh
VII: Bahamut ZERO, Shiva
Crisis Core: Bahamut Fury, Magic Pot
VIII: Diabolos, Shiva, Bahamut
IX: Shiva, Bahamut
X: Valefor, Bahamut, Shiva
XII: Mateus
XIII: Shiva, Bahamut
XV: Shiva, Bahamut, Leviathan, Garuda
Type-0: Shiva, Bahamut, Diabolos