Sona and Soraka are by far the less powerful supports nowadays, maybe only ahead of Kayle and Nidalee support, that aren't meant to be played this way anyway, since Riot destroys everything that is not the way they want.
Nami, Janna and Braum are top tier.
Reason: they can actually protect the ADC while also buffing it. Soraka and Sona only work with low skill people that can't zone decently or get harrassed too much. Also, having a Sona or Soraka at your side gives you the impression of "I don't care if I take damage, because my support can heal me", while the right mentality should be "I will try to deal as much damage as possible without being hit, and if everything goes wrong, my support can stop them".
Janna has a slow, a shield that gives AD, an ok-ish slow and iirc the best knockback.
Braum has a strong slow that can be turned into a stun if his ADC is smart. Also, a projectile barrier that moves along with him that can also be placed in front of the ADC if he jumps right and a giant-ass knockup that slows after.
Nami has a heal, a strong-ish steroid that deals extra damage and slows, an AoE stun and a 2750 range knockup.
Those 3 can stop easily an incoming fed Riven and any other assassin that would burst them down easily.
@Rynozilla
The golden age of Sona.