There's no clear pecking order.

Mesaana had to convince Moridin that Semirhage wasn't trying to kill Rand. Clearly its not difficult for anyone to believe. Its silly to think that these 'levels' even exist. There's a really good chance that none of the female Forsaken know how strong the males are and vice-versa, and is there even any evidence that Demandred has been near to Sammael while he's holding his maximum. Not even WE know if Demandred is stronger than Sammael, we just assume it to be so.

To base Forsaken strength on their statements is insane, unless they directly compare their power, which has only happened on a handful of occasions. Graendal is factually the deadliest of the Forsaken ... She's openly killed two of them, and under other circumstances she could have killed Moghedien, Cyndane and even Sammael.

Graendal wants to 'throw herself on Demandred's mercy' yet she calls him a fool to his face, and openly threatens to kill both Lanfear and Sammael, and she has no problem plotting the demise of the greatest of them all, Moridin.

This pecking order is an illusion, and would only occur if you lined them up in some sort of Mortal Kombat round to see who's stronger. Practically all of them have real chances to best the others. I fail to see how strength levels can create definitive tiers between these channelers, especially when we've seen Cyndane fight enormous odds related to strength, and Graendal prepared to take on both Moghedien and Cyndane. Its like saying that Sheriam is weaker than Moiraine and that makes a pecking order that shows how Sheriam will always lose to Moiraine and 'respects' her because of it. Meanwhile, down the line, Verin could give Moiraine a run for her money and not necessarily lose either.