Assuming a good support player that knows how to actually play and not the random bronze level support players that I get?
I prefer them in this order (in general, without thought to lane matchups, bans, picks, team composition, etc):
1) Lulu Has everything you want in a support except for sustain. Oh well. Great poke, great CS, decent disengage, decent initiate with her allies, overall just very solid.
2) Janna Amazing disruption, great aggression in lane, amazing for teamfights, amazing disengage, amazing passive.
3) Thresh Great cc, disruption, has a decent disengage with ult, can initiate like a boss, better than Lulu. If you pick Blitzcrank before you consider Thresh I don't know what's wrong with your head.
4) Taric Great passive armor, sustain, and initiate boss. Mini baron buff aura wins teamfights. GG, Taric.
5) Sona Dat ult. Dem auras. Her HP change has made it more or less impossible to trade in lane against stronger opponents so she needs a few levels before she can get going now instead of level 1 Q power chord as before. :(
6) Alistar No longer a very common ban and his jungle got weaker but his support is still amazing. One of the easiest sustain supports. In my opinion, much better for sustain than Soraka because of how much he helps in initiation and protecting the team.
7) Leona Initiate yolo support. In certain lanes, I don't know how you're supposed to stop Leona at all.
8) Soraka Sustain and harass queen. But she sucks against teams with heavy initiation and you can often find yourself getting zoned out for 20 seconds at a time if the enemy team is also very good. She seems to be able to do less as the game drags on, which is why I prefer Alistar. At my level, even people who main support aren't really able to save silence for clutch moments or whatnot.
Don't know how an ADC main could say anything other than Thresh, the champ is just so consistently song. Second best for me would be Leona, she's one-dimensional enough that most people can play her and not fail most games (unless you fall far behind in lane).