Honestly its hard to say atm, playing in random 2s meeting people that never actually played healers but even so i fought vs few decent healers in 2s, so depends on comp i might think that priest/druid is the best choice.
So far i would say priest because he/she fears/aoe dispel or even silence being disc and druid because of how powerful becomes one landed long CC, mobility for anti cc and their healing output.
I didn't meet yet any good h pallys so i cannot comment on that.
But i think with less cc any healer with half brain will do ok.
I will play some 3s today.