Is there a list of optimal espers for each character anywhere?
It doesn't work like that. Sometimes you need to swap the esper for the situation. Usually Kilphe's best esper is Diabolos, but you might want to run Ochuu if you're fighting against Orlandu.
For Ramza, I use Two Headed Dragon, but swap to Red Chocobo when fighting vs Archers, you get the idea.
You can check esper board, usually people gonna use slash attack up esper for slasher character, pierce attack up for piercing character, and so on.