I think the best support is the one you can play best. For me the best support is one that can provide a little bit of everything. for example i try and play 3 supports.
Nami i think is the best support for me, her Q, can set up ganks or slow down runners, her W heal bounces off your ally and hits the target as a nuke or vice versa you nuke your target and it bounces off to heal your ally. Her E is a buff.
Sona has aura which reminds me a lot of nami, with utility for her spells and of course her stun ult can stop an enemy engage with the quickness.
Taric, Stun, Heal, and passive armor buff. Can help burst down enemies. Builds like a tank sometimes to.
Lulu is a beast to, but with no heal i dont put her in the top 3 for me. Lulu does have cc though which can own a lane quick. Soraka is all heals no stunn etc so not in the top 3 for me. Leona is good to, but lack of range to me makes her a little tougher to set things up. For me anyways.
Its all preference. All depends on your play style.
I tend to pick depending on who i see is on the other team. If i see a lot of mellee or beefy champions on the other team i go taric. If i see nunu or saraka i go sona etc. Other than that i go Nami for everything else.
Those are just my 2 cents. Even though i like Nunu also, stand in the bush and ult and noone see's it. Fun stuff.