to touch on the eldrad subject isnt he dead? and aren't ahriman and magnus still alive? i must say that eldrad was most likely the eldar's best farseer ever in my opinion, i dont know much about eldar history or the like so my bets in with him, but he's dead now. now for the order the emp is obviously at the top with Magnus not far behind him, magnus was already powerful but then his powers were boosted once he turned to tzeentch. next i think is a tie between eldrad and ahriman, eldrad like i said before is powerful but so is ahriman, to be able to do what he did to a whole legion and have the brass balls to hunt down as much knowledge as possible and even start hunting for the black library means he must be truly powerful, plus he's a chaos marine
other then that malcador would be in there somewhere, most likely above eldrad and ahriman.
Here is the beginning of my fluff for my Iron Warriors Eighth Grand Company https://www.heresy-online.net/forums/...ad.php?t=51842
"A fortress is a living thing: the commander its brain, the walls its bones, the sensors its eyes and ears, the troops its blood, their weapons its fists. This tells us two things: If one organ fails, the whole dies. And if the whole dies, no single organ can survive alone."