All four Forsaken have a reason or another to be suspected of being the "last one standing" in the series, but obviously only one of them will be. Moridin to bring things full circle, as Jordan liked to do. He sure is the most likely candidate, but then there's SH that can play a lot of his foreseen role. Thing is, Moridin doesn't really deserve to die, as the final death is what he wants, so he shouldn't be killed before Shai'tan is sealed off again, and can neither seize nor destroy his soul, condemning Moridin to normal rebirth.

Demandred could stand as the betrayer of his own side (ruining the Shadow's victory by moving against Moridin), but I think he will actually fall victim to Jordan's usual sense of irony: Demandred will be killed by a jealous underling.

Lanfear as she made the Bore (sort of) might survive long enough to see the last of her life's work undone.

Moghedien might be the last one because she might actually manage to hide and survive - but I get the feeling one of the two mindtraps will be crushed by Moridin.

Of course, once Jordan conceived of the sequel trilogy, he might have decided to spare a Forsaken to go join the "murderer" who will rule over Seanchan.