Arthas? Yes, as the Lich King he's over.
Ner'zhul? doesn't look like he's truly gone.
Kel'thuzad? Almost sure he's still there.
What about others?
From the blood princes example we can see the Scourge members can be bring back if the Lich King wants. Or some higher rank undead can do it, too. Dar'Khan was once burned to ashes by the sunwell's energy. But he was brought back in BC with his body, without the Lich King's help.
I guess most of the undead members we killed can be bring back if the Lich King wants. If Ner'zhul gain control of Bolvar's body, then it's like Argent Crusade fought so hard but affected the Scourge very little.