Pros&Cons:
Janna:
+A poor shield with very good damage increase for carries. The shield itself is nothing special - protects a little bit, but not much more (increases with AP though). The main part is damage - at rank 5 it gives 50 bonus damage for 5sec, which is damn good at lvl 9 and still good for the rest of the game.
+Both W (slow) and even better Q (knock up) are nice forms of CC.
+Her ulti is counter to any AoE setups.
-Very late game her damage on shield becomes less effective.
-Her ulti is very tricky. You can win a fight with it or you can easily lose. You can save opponents with it if you don't use it well.
Soraka:
+HEAL. 2x heal with nice AP ratio. One is global, the other one has rather short cooldown.
+Silence. Pretty long silence to counter all spamming heroes for a while.
+Armor bonus on her W heal. It is very good to either prevent tower diving or make tower diving able to you early.
-Gets countered by Execuitoner's calling or some other way of healing reduction (MF, GP, Kata).
-Needs AP to remain effective. Therefor needs items, which results in less auras and less wards.
Sona:
+Early game decent harass with her passive.
+Gives small amount of whatever you need (Auras)
+Very good CC on ulti. Dance bitches.
-The auras are not very useful later, which makes a whole champion abit meh
+Stun on demand.
+Good early healing.
+Armor aura
+Very good pusher due to ulti.
-Mana is a bitch if you need to keep your ulti up
-Heal gets ineffective later
Zilean:
+Imba laning harass
+Speed/slow on short cooldown
+REVIVE
-Bombs become shit if he doesn't go AP
Here's the "support tier list" solely on my opinion:
Janna > Zilean > Soraka > Taric > Sona
I would say Janna is currently doubtlessly best support at the moment. She only needs to come close to fights for ulti. Else she just stays with carry and protects him. Another thing to point out is Soraka. She just seems highly underrated. With a little AP she can turn the tides of teamfight easily. Sona on the other hand just seems like she's lacking something :/
Discuss,
have fun.