I'd say healers but there are actually quite a few. It just seems like there isn't a lot because they are in such high demand they are always doing something. Tanks are indeed the least common, gladiators mostly. I almost never see them in open world but then again they have near instant queues and if I could dungeon spam I would. Ironically gladiators are also the most commonly poor played class. Meaning most gladiators I party with are absolutely horrible. Everytime I get one who can't hold aggro or doesn't mark etc... makes me facepalm and want to roll a gladiator just to see it done right.