Honestly, there is no such thing as the "best". Every support has their pros and cons, as well as amplified pros and cons with different ADC's. For example, Nunu has the ability to turn regular ADC's into hyper carries later on in the game. Soraka and Sona have sustain lanes capable of making the lane phase easy for weaker lane phase carries. Leona, Blitz, Taric have the potential to quickly net kills during the lane and can initiate team fights later on in the game.
So overall, I personally feel that each support brings uniqueness to the table and bot lane comps is a mini game in itself.