I have not played MMOs in awhile but I think healer requires the highest IQ yes, I remember pre healing in raids just because I knew certain retards were going to run in to aoes. Tank is a close 2nd in terms of IQ but probably the most important overall, in MMOs a good tank can carry a bad healer the other way around is more rare, probably more so in WoW where mythic dungeon keys were fast paced. No idea how it is now