Again, other than generic job classes? I'll even include those that can use white magic but aren't necessarily classified as White Mage
Aeris and her limit breaks were super useful.
I think Banon was actually an Oracle, not a White Mage, but I still voted for him. I wanted more of Banon in FFVI. I love his Amano art, and his Health/Pray ability is grossly overpowered (as long as it levels appropriately, which I'm not sure about). Aeris is cool, of course. She has great limit breaks, but the timing is terrible. You have to try really hard to get them, and then not really have any use for them at all. Wasted potential.
I'll throw my hat in the ring for Banon too...his Pray ability is nothing to sneeze at, since (unlike Rosa's) it heals a very significant portion of your HP, and it always works.
That, and Banon is one of the more interesting characters to come out of FF in general. I don't recall another time when you were required to keep a character alive, and the reasoning for this ties into the story nicely (i.e., if Banon dies, the Returners are done).
Well if you are going to include Cecil then I have to pick him. But I don't know that I would consider that a valid choice.
In terms of White Mages as characters I can't say I every become hugely attached to any of them. In terms of usefulness I would go with Eiko, Yuna and Rosa from the versions where she has 'Pray'. Given that a large party of why I find Eiko and Yuna useful is summons it kind of seems unfair to chose them over Rosa, who is pure White Mage.
EDIT: Cool, I can vote for multiple people for whatever reason.
>>Am willing to change opinions based on data<<
I have to say Yuna. Auto-Life, and Holy are very useful especially with Nirvana. Plus she can summon!
"geroeen, pulsa nyet, tolska tanee pre chem"
"Beauty is a perception tainted by ego;
Love is an extension of great emotion;
Fear is the result of uncertainty;
Sadness is a collection of misery."