Thresh is highly considered the strongest support right now, with good reason. Lulu is probably a close second. Taric lost a lot of power and isn't being used as much as he was before the last nerf. Leona and Sona are also very popular but they are not as powerful as Thresh/ Lulu.
Support generally are doing well if they aren't dying much and have double digit assists. However being a good support goes beyond that, how many wards you place and where. Generally being in the right place at the right time and helping your team. Supports have arguably the most work to their job with the least respect half the time, right next junglers.
Also, it should be noted that the best support choice depends on your carry/team. for instance:
Thresh: Works best with aggressive carries ( Draven, Graves, Miss Fortune), especially ones who lack an escape method, which he provides. He also works very well as a team initiator with his Q and ult.
Lulu: is better for "protect the carry" comps and works wonders with late game carries who need to live as long as possible like Vayne and Kog'Maw due to her shield and ult. These things also make her great with initiators like Xin Zhao or Jarvan IV.
Leona/ Blitzcrank: when your team needs some hardcore CC and a tanky frontline and desire to kill things as fast as possible. more aggressive and less safe than Thresh but in soloq, catching people out of position with these can win fights so easily.
Sona/ Taric: All around safe picks and do well with pretty much every comp.
Dont bother with Kayle as a support. Nunu & Willump is kind of the same deal as Lulu in that hes great for late game carries. [{Alistar]] is iffy, but can be great if you get the combo right.
Used Taric i don't have many support champs.
I have allistar, Nunu & Willump, Taric, Kayle. Lately i'm pretty much forced to support becouse of "mid or feed" and similar ******s...
Support generally are doing well if they aren't dying much and have double digit assists. However being a good support goes beyond that, how many wards you place and where. Generally being in the right place at the right time and helping your team. Supports have arguably the most work to their job with the least respect half the time, right next junglers.
Also, it should be noted that the best support choice depends on your carry/team. for instance:
I have allistar,
I think scorewise, you should always try to have at least twice as much assists than you have deaths.
With champions, that would probably be:
Taric, Leona, Blitzcrank, Thresh, Sona, stuff like that :P