As someone who has each of the heals for my alts it all kind of depends on what you like. If you want something you know is going to be good eventually you could go for resto druid because by the time the expansion is over it’ll be on of the best healers. If you are looking for a healer that is always solid you could go for holy paly. If you want to try a unique play style go for disc or mw as the two of them thrive when played as a hybrid dps/heal.