Assuming all are at full power. Prime WotLk Lich King with Frostmourne. Malorne fully recovered and energized. Hakkar the Soulflayer having a shitload of troll sacrificed to gain a power boost. And Ragnaros being in his true legged form. Who would win in a 1v1 duel? Battlefield is on neutral ground. No interference or minions allowed. Also who do you think has the potential to be the strongest?
Malorne the Forest King
vs
Lich King the Lord of the Scourge
vs
Hakkar the Blood God
vs
Ragnaros the Fire Lord