all healers r rly strong atm
druid hots heal instantly as well now, and swiftmend doesnt consume rejuv/reg
priest does sick dmg and healing as well as 1k bubbles and mc/psyfiend/fear cc, best class in 19 atm
shammy has lots of abilities, insta gw, both dispel and purge, windshear doing a 3 sec lockout now rather than 1 sec, riptide, healing surge, so much stronger than they were in cata and on defo a great healing class despite not having the damage disc has
pally still strongest heals while hardcasting, holyshock heals nice as well and their mobility is just insane, +30% movementspeed with 3 holypowers, great for fcing
Sums it up pretty good, a few more points I would like to mention though:
Druids also have 2 rooting abilities now (bear charge from talents, and obv entangling roots), which are very valuable for a defence, especially when there's a lot of melee classes after your FC. The roots become more valuable by each debuff stack, especially when rogues + ferals crit shitloads now (fero bite with savage roar + tigers fury crits around 900 in mail geared hpala, and 1,5k on clothies.) It's crucial to keep ferals off your FC as much as possible, since their single-target damage is probably the strongest in MoP. It's hard since they will just shift out, but manageable. Rogues are obv a lot easier to keep off.
Also, with druids having the ability to shift out of roots + slows, and shift before poly hitting them, you will pretty much never fall behind from your FC. Oh, and you also get a leap to friendly target from talents.
Shammys utility in defence is just huge, and to add what Hurrxiboi said, ancestral vigor is also a very solid buff for a shammy even though you would think so. 10% buff to max hp (which I think it is capped at?), can be close to 300hp on a well geared FC, which is very nice to prevent those rogue groups rapes at higher stacks.
Priest does huge damage with penance, biiig shield and good CC with fears + chastice (if holy). Very good class but can be a bit squishy if targeted by many melees, seeing as it's still a clothie. Though, it is still only healer capable of doing viable damage. Good choice for an offensive healer.
Pala has strong heals + armor, and melees will have a rough time downing one. The movement increase is always a nice bonus, and with HoJ you have a little bit of offensive utility aswell. Too bad that they removed exo, so pala isn't capable of doing much damage anymore.