The QP I submitted asked who the 2nd most powerful Forsaken is, after Ishamael. The two most popular answers were Demandred and Lanfear.

Jordan named Ishamael, Demandred, Sammael, Lanfear, Graendal and Semirhage as the top Forsaken in the Shadow - who were always near to the top of the Shadow hierarchy due to their skills in the physical and political world.

My own personal theory is that these six Forsaken are pretty similar in the One Power - so similar that dueling will probably lead to unpredictable results. I don't think that they are similar in strength - Ishamael could be considerably strong in saidin compared to Graendal or Semirhage. However, they all have special skills which make them powerful Forsaken. None of them hid like Moghedien, and all of them have shown that they are prepared to openly challenge a threat of equal power.

All of those six are extremely powerful in the One Power, with Ishamael and Lanfear showing the highest possible levels attainable, and the other four with slightly less. Regarding individual skill, Demandred is generally skilled in most things, and Graendal and Semirhage are among the most skilled in their fields who have ever lived. All six are equally willing to kill an opponent, and are all established mass murderers. Lanfear, Sammael and Semirhage are openly aggressive Forsaken who devastate first and ask questions later. Ishamael, Demandred and Graendal shy away from direct conflict if they can and use precision strikes and uncommon skills to undermine their opponents.

In summary I think that those six Forsaken are close to the top of everything - strength, skill, attitude towards battle. Where one has great strength in something it is balanced by a weakness somewhere else. Ishamael is probably the single most powerful of the thirteen overall, which is pretty much hinted at in the books on several occasions. The other five are probably all on a relatively level playing field.

Demandred, Lanfear, Semirhage, Graendal and Sammael could probably duel one another with unpredictable results, and the result would be 50-50 if they dueled a hundred times. It makes them inferior to Ishamael and Lews Therin, but not to each other.