There is no best, it is completely based on match-ups and players.
If you're still looking for a "best one", the closest we can get is Janna.
She was one of the champions that I earned the most ranked wins with in Season 2. Basically, Janna is in my opinion of the best because all her spells have an immense amount of utility.
First, her passive is one of the strongest in the game - 3% movement speed globally, for all allies! Seeing as 3% MS is worth 2 MS quints or 21 points into utility mastery, it should not be any wonder why this is insanely powerful.
She can push, disrupt, harass, peel, and chase with her tornadoes and zephyr, which is useful for the ADC in lane and generally the whole team later on. It gives you the ability to harass/push when needed, but also to help protect teammates or kill enemies. Even better, Zephyr gives extra MS while also making you ignore unit collision, which again, is very powerful. There are MANY times where this comes in handy, and one of the best things about Janna is she has such a speed advantage over everyone else, it allows you to do things you normally can't do, and makes it so much easier to plant wards. It makes it less punishing if you get caught out, or if you're far away from a teammate in need.
The shield is one of the best support abilities since it also doubles as an attack steroid. Then there's her ultimate, again a versatile skill. Can use it to push enemies away or towards team for initiates, peels, escapes, etc. Or, it can be used to heal a team to keep a push going, or give them enough health to take an objective like dragon.
Sure, she lacks a spammable heal, but I don't consider this a real weakness, and it certainly does not offset Janna's strengths. With the S3 changes, it has become easier for teammates to heal (Flask), and since pretty much everyone has some sort of anti-healing mechanism (Ignite, certain abilities/items), it's not a big deal.
Again, there is no best support, it all depends on the players and match-ups. However, Janna is one of the safest choices in the game.