Elle

Elle is absolutely amazing. Pick her once, and even with legendary gear, she'll carry you through most of the game. Initially you might not feel like healers play a big role, but they're indispensible during the late-end game. 
Currently, she isn't the best healer in the game because Eline, Reny, and (most importantly) Wedding Dress Lin have fantasy upgrades. However, she is so good that her healing at Mythic rivals and sometimes outclasses the healing of Fantasy-grade healers. 
Most of this comes from having heal/barrier focused skills with a 20% crit rate (other healers have 10%).
Note: These are skills at lvl5, not lvl6.