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

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

http://www.lolking.net/champions/
Sort by popularity to see which supports are most played.
Sort by winrate to see which support are most viable. The winrate backsup this statement.

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:

• A lot of CC.
• High base spell damage.
• Early game harassing potential.

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