Odin: Esper with the deepest backstory which mirrors the main plot, And if I recall he is the only way to get +speed in the snes version
Maduin: He's a love machine
Yura: Loved the name, loved the sprite, shame we never got to see his battle powers.
Ifrit: Good old classic esper, considering the importance of espers in this game I do wish he had a bigger plot role in the game.
Shiva: Same as above.
Ramuh: As much as I love his role in the game, an esper that just looks like a human feels too strange for me.
Esper village Elder: STOP TRYING TO BOSS ME AROUND

Feel free to include anymore in your rankings if you want to bother with the minor npc's of the esper world or indeed if I have forgotten any important ones.