Formenos- You put it first in the list. It's gotta be it, cause that's just the way you are...

Ok, and it doesn't fit the rest of the criteria
Angband- Fell because of Earendil's (who was half Man) plea to the Valar for help
Gondolin- Many of its people were saved from its sack by Tuor
Nargothrond- Was destroyed easily because of Turin's decision to build a bridge
Tol-in-Gaurhoth- Was destroyed for the sake of Beren's escape
Formenos was the only one whose downfall did not concern or involve a Man.