The Sybil sounds very cool fluff wise... but I haven't actually read it yet...
The knights of Nirvana need to get made, but at the same time don't seem to be very interesting... would really need to flesh out the "inner death of self" angle to give it bite for the feel of the characters.
Children of the Mausoleum would be the most daring, and thus probably the best, if it required a GOOD alignment...Note that in such a case it would be an interesting option to have them consider mindless undead, ones that change alignment, and to a lesser extent ones with a "hunger" that can only be sated by harming sentient to be violations of undead... give them the ability to grant free will, breaking all Command undead, bloodline command (IE being under control of undead that created them) and magical compulsion on undead in lieu of standard Turn/Rebuke, and you might be on to something interesting... they come surrounded by slightly timid free-willed skeletons who serve out of gratitude for being released from slavery... they skeletons might or might not be willing to fight for them, but who needs a Bag of Holding when you have 10 tireless back(bone)s to do the heavy lifting for you? One way would be to basically have them grant/convert pre-exist class levels that had/returned to the undead they work with into levels of Emaciated Spawn*?
I have only heard of this PrC so I might have the name wrong... could be EMANCIPATED Spawn...