"Immortal Coil" is wonderful, although when some TNG-only readers found out how much continuity came from TOS they got very angry. I still think it works extremely well as a standalone.
"Metamorphosis" but, again, it made some fans very angry. Fans are funny like that. I does have some follow up with original characters from "Survivors", the early novel that ties into "Skin of Evil" at its conclusion.