RDM

Used to main WHM but, as you can probably see from the picture I uploaded of the two classes side by side, RDM is a much more powerful and responsive healer.

With an effective 350 cure in 1.19 seconds average and the option of rezzing 3 party members within 10 seconds there's just no other healing class that comes close.

Area heal is beyond RDM, and MP can be a major factor as practically noone uses Manashift (hell, most SMN's don't even know they can rez until a healer shouts at them in the Ozma battle).

The best thing about healing as a RDM is hearing Reddit-junkie tanks whine about dps loss...there's more dps lost from healers healing than RDM's healing most of the time.