I would say that there is no "most" powerful and these guys are all pretty much evenly matched.....

-Mannfred von carstien (The guy is nutty powerful now in the fluff, I call him Nagash Jr)
-Nagash (Kinda but he keeps getting WTF pwned...)
-Neferata (Angry female queen anyone?)
-Malekith (Didn't he steal nagash's crown?)
-Morathi (Dark elf chaos blessed crazy woman!)

Oh wait we all forgot about

Valkia, a gods consort......ya safe to say she is going to take on everyone and win.