So who's on top? Last time around, we said Sargeras or Elune, but let's refine things a bit-- those two may both be very powerful, but Sargeras is currently "ceased to be," and Elune, who supposedly created the world, doesn't actually use that power much, does she? If she was so peaceful and in charge, why's the world called Warcraft?
Still, looking down the list, Elune does seem to top it-- if you're talking about potential power, she's it. Aman'thul and the Pantheon are up there-- they toppled Sargeras and have even killed the Old Gods. But the most powerful character that we know for certain is actually in Azeroth right now? Besides the winner of the our One Boss Leaves tournament, I'm not sure. Nozdormu? The Lich King?
Unless by "most powerful character," Killah actually means player character. In that case, I have no idea. Awake? One of the guys from Team Pandemic?