Discipline dominates nearly every fight because of the way raid damage works:
HC Stone Guardians? Spirit Shell can prevent a wipe here even if you mess up. No other raid CD (outside of a stacked raid under PW:B lol) or AM/Rallying can do this, but Spirit Shell's CD is so small it allows you to totally play like mongs and get through the fight.
HC Feng? Arcane Velocity vs Spirit Shell, Spirit Shell wins.
HC Garajal? Atonement is very strong here. It's how most guilds did their first kills: 4 healers with a smiting Disc Priest for the entire fight for a) added dps and b) solid healing on those who need it
HC Spirit Kings? Granted, Disc isn't "that strong here" but it's still among the top 2 healers (monks/pallies tied for #1 spot IMO)
HC Elegon? Monks > Discs on this fight, but that's because Monks dominate p2/p3 due to Eminence.
HC Will? Spirit Shell prevents wipes.
HC Vizier? Spirit Shell vs F&V? Spirit Shell wins.
HC Blade Lord? Spirit Shell makes the only hard mechanic (Unseen Strike) negated.
HC Garalon? Discs tied with Monks here, Disc is really good on this fight though.
HC Windlord? This fight is gimmicky so badly, it's dumb to even use it. Monks do well, all classes do. Atonement is strong because it's 600% stronger.
HC Ambershaper? Disc is strong here. Spirit Shell can mean you can survive a few bads messing up with the construct.
HC Shek'zeer? Spirit Shell is really, really strong all over the shop here. The only "negative" is that Disc Priests take a lot of damage from the Sonic Explosion due to low armour values.
HC Protectors? Discs raid CDs help so hard here and will be mandatory (stacking wise) for heroic elite, this is common knowledge.
HC Tsulong? Monks own this fight. We have all our Priests go Holy for Guardian Spirit: Holy performs really well in the Night phase though.
HC Lei Shi? Disc does well here, but the fight is a joke. It's easy.
HC Sha of Fear? Atonement is king for healing. Absorbs can negate *almost* all thrashes/dread thrashes.
Disc is strong on every single fight. On Most Disc is ridiculous.
Signed, someone who's done 16/16 HC: http://eu.battle.net/wow/en/character/frostmane/Nymzee/