FF4 has my favorite summons, not necessarily because of their battle prowess, FF3 probably wins that for me due to busted ass Sages, but due to the narrative elements involving them. I remember being a kid and killing the mist dragon and being all, 'Nuuuuuuuu! T-T' afterwards, then titan, what, spawns a mountain range in your face in response? That was pretty awesome. Finding the Feymarch later on in the game basically seals it for me, since you see the summons have their own little society underground, and Rydia was hanging out with them for all that time.

Espers had their own village in FF6 as well I suppose, but, and frankly this may just be my hatred for the Maudin section slowing the game to a halt for that five minutes I stopped caring about the third time through the game, I think the place is boring as all hell and you don't have any real interaction with it, and none of the other games in series really do anything with the summons that leads me to especially like them at all -- they are basically just (usually outclassed) nukes with long run times.