In my opinion-
DPS (spec wise)
1. Balance Druid
2. Surv Hunt = Warrior = Rogue = Arcane Mage
3. Ret Paladin = WW Monk
4. Warlock, I guess?
Healer
1. Disc Priest
2. MW Monk = Resto Druid = Resto Shaman
3. Holy Paladin = Holy Priest
If I've missed anything (like feral druid) it's because I really haven't seen anyone play that class/spec. Also, all healers are good in their own way, but if I were to have to rate, this is how I would.
Edit: Also, I don't really remember who it was who said this, but someone said that MW was like resto shaman but worse, and I disagree. As Fat Al said, MW is super versatile, and I'd argue that their potential burst is easily better than that of a resto shaman. Only downfall is that resto shaman damage is from a range, MW is not (besides crackling jade), but it's still amazing for group fights and I feel as though if someone is running a premade v premade (not like that ever happens), a MW is a must have.
Not to mention, their burst heals are meh, but once you get Enveloping Mist up on someone, it's very hard to drop them unless a shitton of people are on that one person.