The meta changed significantly after this question was asked. 

For the season 4 meta:  

 - You do not want a support that heals.
 - Support that buffs generally not preferred.

You want a support with:  

 1. A lot of CC.
 2. High base spell damage.
 3. Early game harassing potential.

Lets see what Taric has:
 - No range harass.
 - Long cooldown, high mana heal to himself and ally.
 - Armor buff to himself and allies.
 - Long cd stun, with insignificant damage. 
 - Not bad one time burst with really high cooldown.
 - Damage buff to allies.


 - Tarc doesn't have any of the stuff we are looking for in a support. So he isn't a viable support.
 - Soraka has nothing at all. At least Taric has meh cc and meh damage.
 - Sona has: 1,2,3. 
 - Janna has:1,3

So if we compare these,
Sona>Janna>Taric>Soraka