Sona - get caught? instant death early game, but great poking still and sustain
Nunu - makes any semi-competent AD into a badass. Great ult, helps with controlling objectives, shuts down other AD carry for short periods of time.
Taric - Great laner, not as good later on.
Nami - Lots of potential, but gonna have to see her in actual servers before commenting
Leona - Good, but if her team falls behind/she doesn't get kills she becomes sub-par late game
Lulu - As the tournament proved she was pretty much one of the most useful supports and has an awesome skill set that can troll things with long windups like a Darius ult or Nocturne even. Her ult is a lifesaver, her poke is great, she's got nice CC, etc.
Blitzcrank - Land 10 pulls, miss one. "Wow report this **** Blitz." Alternatively if you pull the wrong one...
Alistar - I feel like if they stop banning Alistar out, Ali could potentially be an awesome support. With his ult on, there's pretty much no way for him to not be effective in the hands of even a semi-competent Alistar player. His CC is massive, great peel, sustain, etc.
Janna - nerfed too hard.
Soraka - Support 101 is support 101. At least she's prettier now.
"When you want to be successful as bad as you want to breathe, then you'll be successful!"
I would have to say Ali or Blitz, just because of the great hard CC. That makes them so effective throughout the game, but I love Sona and Taric as well. Nunu is a situational pick, but he can support from the jungle as well, giving a double support late game (usually has to build tanky as well to be effective).
Violence is never the answer. Because the answer is always Vlad.