I have done M+ with Monk(853), Shaman(846) and Druid(845) and have to say it differs a lot between classes and situations. Over all i would say its (From Mana-sustain downwards) Shaman>Druid>Monk. On the other Hand, the mobility (which provides time to drink, while your party is still running to the next pack) is Monk>Druid>Shaman... IMO its fine, you only have to accept, that you need to drink very often for a few ticks with some classes... I just sit down when ever i can (even at 80% Mana...), just to not fall back in mana over time.