That really doesn't hold weight when FFX added Yuna and yet Wakka is the most powerful party member and Rikku is the most versatile.
Same with Tifa when Barret and Vincent had the Overflow glitch and Laguna over damn near the rest of the FF8 squad?
Except in the case of FFIX, Quina and Amarant (Salamander) were the most powerful and versatile party members in and out of Trance, with everyone else being situational at best.
Zidane was an average character at best, and was particularly useful in stealing useful items from bosses. In battle, he's an average damage dealer that's naturally better in Trance (with damaging Dyne specials to boot) and requires finding all the treasures (including sidequests) to utilize his "free max damage" special Thievery.
Freya's a great asset early on with skills like Reis's Wind and White Draw, but even in Trance she's an average damage dealer, with her best offensive option being her "free max damage" special Dragon's Crest, which requires killing a lot of dragons to maximize its damage.
Steiner is as straightforward as it gets, with occasionally useful skills like Charge when your party's in a pinch. If you're in Trance, #freedamagefuckyeah
Vivi's black magic is only as good as often as you use Focus.
Dagger's a bit on the weak side in white magic, but her offensive summon magic is decent. However, to utilize it best, you must have her in Trance and have her summon an Eidolon only once. If you skip her turn, she'll use the recently selected Eidolon an infinite number of times without ever using up the Trance gauge. The only downfall is that she's unable to act, and the trick resets when she dies.
Eiko by comparison is easily the best white mage of choice, but like others in the cast, she leaves some things to be desired that both Amarant and Quina provide. Chief among them, the ability to use Auto-Life, utilize guaranteed instant-kill options for stronger enemies, and best of all, a fighting chance against an army of 3 Yans - the most powerful non-boss characters in the game - without getting a party member ejected.
Rant aside, I never said nor intended to imply Eiko was overpowered. I only mentioned she was more useful (as a white mage) and had better leadership qualities than Dagger. Sorry for the misunderstanding.