My very first Knights of the Round summoning just left my jaw open. This was my first RPG where damage was estimated by numbers, so seeing all those numbers jump out of the poor Kalm Fang was just amazing.
I hate Ifrit, his damage is just too low at that point in the game. Shiva is useful against the first Jenova form at least.
OTT, probably Kjata. The sequence was long and complex and yet the damage he delt was moderate at best.