good question
warden has by far the strongest singletargetheal (also quite a few hots too)
summy has as far as i saw the strongest aoe healing but has to stand still while doing it so not rrly useful for pvp if you ask me but pve its fine (but probably weakest healing overall of the 3 healing classes)
cleric heals less than warden but also has the ability to give shields which some ppl prever over heals in pvp
so in general you can say if you prefer pve take warden if you also want to do pvp maybe better take cleric (even tho warden can easily heal in pvp too since iam doing it
also next patch warden gets a boost and can provide shields too)