First Playthrough: All the Bahamuts, Hades, and couple more I can't remember. I didn't get KoTR the first time I played so I obviously never used them.

2nd Playthrough: KoTR and Hades. Since I got pretty much everything in the game this time through. Summons were barely needed, especially when I dealt out more damage from a single "attack" than most summons provided.