I've healed on all three in t11, my priest is my main. Druids are nice because they're highly mobile and have a lot of utility, but I find that they don't have as much burst as my priest for things that don't necessarily warrant tranq (if you don't have hots rolling on everybody already, that is). Shaman are cool because they have totems, but I sometimes feel "boxed in." I love my priest because I feel like I have a tool for every situation. That being said, that means it's more difficult to know quickly and effectively which is the best for any given scenario.
As stated so many times, the choice of a class is a personal decision.
13.6.2. Pointless threads
"Which class should I roll?" is a frequently reoccurring thread.
There is no real discussion to be had about which class any individual should play. This is a very personal decision and all anyone can really do is post their personal favourites over and over again.
If you wish to find out more information about specific classes, feel free to browse our Class Forums. You can also try to use the forum's search function if you have a specific question about a class.
Other examples of pointless threads include, but are not limited to:
Which race should I roll?
Which faction is better for me?
Which is the best PvE/PvP/... realm?
Looking for RAF partner
Pick a class, and try them. All healers can heal in 5 mans and raids. It doesn't matter which class.