Soraka would be good. Except they've nerfed her constantly since her slight rework. Just after it. She went from being nothing but a heal bot, to a more versatile mage, being able to support via MR reduction, silence, constant ap damage, and denial via the high CD powerful heals.
The only problem is, Riot nerfed this several times. Infused was nerfed twice, Starcall was nerfed twice, Blessing was nerfed 3 times, and her ult was nerfed once. They just kept whittling her down until there was nothing left by an impotent support.