Generally speaking, the Espers get stronger as you acquire them, meaning Belias is the weakest and Zodiark is one of the most powerful.

It depends on how you use them but my personal favorites are Exodus (Non-Elemental) who uses Comets to destroy enemies. He's a bit slow in his casting so you may need to use your summoner as a meat shield. He complements melee fighters well if you set everything up correctly.

Chaos (Uses all the elements except earth) is overall, really good and his Gambits are set up to exploit elemental weaknesses. He kicks ass and sadly gets overlooked. He's good for any kind of character.

Ultima (Holy Elemental) her ultimate cinematic attack is damn near impossible to get off but her normal holy attack hits as hard as Holy and is much faster. She has auto-reflect so magic bounces off her but it creates poblems when healing is needed.

Zodiark (Darkness/non-elemental) has the easiest and most powerful Ultimate attack to get off and is overall good but seriously, you just want to set him up to be a classic summon where he appaers and casts his Ultimate Attack.

Hashmal and Famfrit are also nice cause they exploit two elemental weaknesses that you either don't get direct access to like Earth(Hashmal) or never get anything higher than the basic lv.1 spell like Water (Famfrit) and these two do serious damage. Their elements are also uncommon so they stay pretty good even by end game.

You do need Belias for later and a few of the others are still good but these 5 I feel are the most abusive in power. Unfortunately, with the exception of Hashmal and Famfrit, the four two can only be acquired through doing the sidequests and battling them. Ultima and Zodiark are easily the two most frustrating battles in the game if you you don't have a strong party.