6* Tilith may unquestionably be the best healer in the game until Ayaka comes out, but it's Orlandu and enhanced Ramza that create the big jumps in power, not Tilith. And it's also not Tilith's 99 that makes her so good, it's the strength of her entire kit as a whole. She has super camo, curesunaja, aoe break immunity, aoe arise, aoe full heal, dispelga, aoe resist all (except dark), on top of her curaja+ with mp regen.
You'll be using her full heal every turn on stuff that's really healing intensive anyway because prism heal (the mp regen) doesn't affect herself.