I'd say Soraka just because the passive meta is pretty much dead and gone, Poke and CC supports rule the roost now. Nami would be a close second but just because she requires a lot of synergy between her and the ADC to use her skills effectively.
Nami has a really mana-intensive heal, a decent ult, an E that's really pretty lame, and a barely noticeable passive
she just doesn't do anything well enough :\
Her W isn't that bad if you use it properly, like pretty much all her kit you're meant to use it in trades, not just throw it out whenever the ADC gets hit. Her problem isn't so much that her kit is that bad it's just other supports can pretty much carry bot while Nami is reliant on her ADC being half decent.