The Shivering Isles hands down. The Knights of the Nine was everything Isles wasn't:
Anywhere in lore before this
Never referred to again in the rest of the game or sequels
The Knights of the Nine had promise, I'll give it that - going to take out some kind of demon which sacrifices priests; going on a pilgrimage like a real RPG; getting together an elite crack squad of templars and solving puzzles like Indiana Jones and the Last Crusade. It's really fun to play through but the outcome is so pitiful that you're left with a priory to yourself and some magic items (some of which can't be used by certain archetypes and skill users, I might add). It wasn't bad, but it lacked so much replay value and I only do it still because I paid for it, plus it's different to other quests which makes a nice change
I've reviewed both pieces of DLC for a more in-depth review, but the bottom line is that Knights of the Nine pales in comparison to the art style, morale dilemmas, fascinating cast, memorable side quests and final outcome of Shivering Isles. When I can say housing DLC was more memorable with a straight face, that's bad.
Shivering Isles review:
Knights of the Nine review: