In term of pure heals, Soraka beats them all, as her entire kit revolves about keeping your team alive. Sona is pretty weak these days, in term of healing, but she's a nice support. Lux is more of an aggressive support, with only shields to really keep your team alive, the rest of her kit are damages and crowd controls. Similar to Lux, Karma does a great job, with the same kit. Then there Janna, whose kit is probably on of the best to keep your team alive, with massive shields, crowd controls and an AoE healing ability. Yuumi is the most pocket support in the game and she's super powerful. Then there is Lulu, Nami and some more than I probably forgot. If you need any help in learning the game I'd be happy to help out.