I think it depends what instance you are in.

If we are talking about raids...like savage/more serious raids, it's going to be healers (imho) hands down. It's a twitchy job to play in the harder instances. Not only do you need a good grasp of the job but you need to plan ahead AND need a speedy connnection AND a good control scheme to execute your heals.

Farm dungeons for tomes, DPS are the mvp and only 2nd to that would be a tank that can do decent threat. It's a combination of things to be sure, if you can get a healer that can also feel comfortable to dps too, that is a big plus. But dps who know what to do and aren't jerky about it by pulling early shine here.

Pugged trial roulette and random primals? This is where I say good tanks shine. No one likes to really talk too much in random pugged trial/primal fights. Special bonus if two tanks can figure out who is main tanking and not fight over the boss without even speaking.

That's just a rundown of my personal feelings, your mileage may vary.