I agree more with what other people say.
VI has the whole planet being taken over by a maniac raining destruction (Why Kefka>Sephilame)
Tactics has betrayal and its effects.
However, I've always found IX the darkest.
The amount of death and its consiquences in that game is more than the rest of the FF's. The likes of the destruction of innocents at Cleyra topping it. Although, the aftermath of the Lindblum attack is pretty grim, considering you get children crying cause they saw their parents die in front of them