Holy and Disc can both fill the role, especially with the Changes to Chakra: Serenity. Holy's Single Target Healing was notably improved, as well as HW: Serenity being a crazy mana efficient heal.
That said, Disc is generally more efficient and a better option, despite Holy being able to do the job well. Between Penance and Power Word: Shield, you also now have Spirit Shell which can easily prepare a tank for an onslaught of damage with zero overhealing. Also the changes, to Inner Focus now make Disc able to respond to a big damaging tank attack with ease.
If fights allow for weaving of Smite/HFire during periods of low damage (and likely still heal the tank via Atonement) you can also use Archangel easily.... or at the least you can prepare by simply using holy fire to stack Evangelism. You won't be spamming smite nearly as much as you had in the past due to the mana changes, but it is still a viable method to heal the tank when more traditional heals are not necessary.