I haven't gotten to play all the versions of Final Fantasy so I mostly only have FF8 and Crisis Core to go by but of those games I have gotten familiar with a few summons and I have a few favorites, mostly just for the look as I don't really know how much better they are at damage than the rest. That said, my favorites are shiva and bahamut, because they have similar aesthetics and it's this type of power animation that really appeals to me.
What are your favorite GF summons?
What are your favorite GF summons?