Which is easiest for a rookie healer?
Sage is considered to be the easiest healer, Scoundrel - hardest
And above all, which is most fun?
I love healing as Scoundrel. As I said, it's the hardest one, but if played perfectly, shows insane power in PvP.
I agree with the rest, but what makes you think Scoundrel is the hardest healer? Resource-wise maybe, but once you actually learn to control yourself and use the proper abilities and Cool Head, it's mind-numbingly easy. Part of why I went back to DPS after speccing at level 47; it was just run and hit instants, trying to generate as many Upper Hands as possible, or take cover and cycle Underworld Medicine and Emergency Medpack if you're not being chased.
It's a lot harder to stay alive as a Sage, which makes the class harder to play in my opinion. From a PvP perspective of course, since PvE boils down to finding a resource-neutral or -positive rotation and using that, then burst-healing when you need to and recovering energy in a lull. Nothing really reactionary or difficult about developing the best rotation for your class and yet, with the Sorc/Sage nerf, it's still harder to maintain energy on a Sage than it is a Scoundrel.