Most arguments for Eiko seem to have something to do with her being the better White Mage, and while this is true, I typically prefer Garnet, since she seems to have better Summons.
Eiko does have a few interesting Summons herself though, which are more focused on defense.
I have always found the part when Dagger loses her voice to be incredibly cool, I have never seen a character in a game become so traumatised that it affects their battle skills like that.
Sure, I have seen several characters get injured and handicapped in various ways, but I have never seen anyone else than Dagger become psychologically affected to the degree that they lose their ability to talk and to think clearly.