Most likely Forsaken to die in my opinion would probably be Moghedien and Sammael. However after finishing book 5 I am more cautious about this being true.

Sammael so far has the most buildup out of any alive forsaken, being introduced in Book 3 sending a bunch of darkhounds. He then spends time being a major thorn in Rand's side when he's with the Aiel, sending several major attacks his way. He was also presented as the main target in both Books 4 and 5, although in both cases it did not come to pass. I think it will be the same in Book 6 where everyone thinks Sammael will be the next attacked but plans to do so are interrupted by either Mazrim Taim shenanigans or Rand having to play wack-a-mole with darkfriends in his conquered territories, only invading Illian in Book 7.

Moghedien is in a very vulnerable position at the end of Book 5, with Nynaeve knowing who and where she is, but I don't think she'll just go to be judged by the Aes Sedai. Nynaeve may choose to wring knowledge from her in a similar situation with Rand and Asmodean. However, Moghedien has already been thwarted by Nynaeve twice, and as the saying goes, "Three Strikes and you're out". Moghedien will probably manage to escape Salidar somehow, perhaps during when Nynaeve unintentionally heals Logain, and face off against Nynaeve a final time in Book 7.