It's all about your preferences. I see them this way:
Druid - great raid heals, great flexibility, especially wolith your second spec. Only class in the game that can fill all four roles
Shaman - strong raid healer with excellent tank healing support. Unique play style with the utility of totems and ofc, heroism
Priest - the thinking man's healer. Has the most healing tools which can make them the most interesting healer. It also tends to make them not the strongest at any one aspect of healing.
Role what you like.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng