This will all be from my experience in raiding generally, tanking a bit myself and talking with a few tanks.
So far, it seems that a good BrM Monk is the best tank you can hope to have - BrM has good active mitigation (Shuffle, Stagger, etc.), very good passive mitigation, nice mobility, the best AoE tanking, some massive self-sustain CD's and to add to that, they have the best DPS out of all tank specs. The only minus is that it has a higher skill cap than other tank specs and has low armor, no parry. But the positives very heavily outweight the negatives.
After that I'd rank Warriors as 2nd - extremely good at taking melee hits because of high parry/dodge/Shield Block, has some of the best raid cooldowns, amazing mobility and very meaty, simple damage reduction CD's. A big minus is that, because of vengeance not working properly, Shield Barrier is kind of bad making them a bit less valuable on high magic damage fights. All-in-all, warriors are always a solid tank to go with.
As 3rd I'd rank Paladins, though they can easily be 2nd, I'm just more biased towards warriors. Paladins have very nice active mitigation, very amazing raid cooldowns, Hand spells, Bubble and big self healing, a cooldown that basically rezzes and handles all damage types fairly well. Minuses would be that their mobilty is sub-par but that's literally it. Again, a great pick for any group.
As the last 2 picks I'd rank Druids and DK's.
Druids - highest health pools, occasional huge heals, nice raid utility, a combat rezz and they're generally just a big meat shield. There are quite a few minuses though - they're generally not good at taking melee hits and they have probably the worst active mitigation kit (which is the way to go in MoP).
DK's - has insane self healing, has a lot of absorbs, a combat rezz, really good magic damage mitigation and some nice cooldowns (like Anti-Magic Zone). Though they suffer from really low AoE without Dancing Rune Weapon and probably the lowest mobility out of all tanks.
Even though I've said all this, this is just the opinion of one player so take it with a grain of salt.