I'm in the same boat as undecided on which is the best. I'm currently in the process of leveling up a monk and have a warrior not far behind which would give me every tank at 120. I'm hoping then I can make a fair assessment. Like Mirai said though Paladins have always been a good choice for boxing, they give you utility + a strong tank.
For healing Shaman is really nice because you can spam chain heal and not have to stress too much on who gets it. Monk is good because we lose some value with how soothing mist works and it breaks follow. Monk for melee healing was awesome when I temporarily tried it, but concerned it won't work well on some boss mechanics.
Druid is a really strong healer and some boxers have been able to do well with it but I haven't found it myself.
In the grand scheme of things any of the options will work, It also depends on your preference for how you like your tanking/healing. Re-active or preventative. High dodge squishy tank or takes a lot of hits but low damage.