Thinking, not from a game level pov, or from stamina or differculty level, of the end game bosses we've encountered, who do you consider the most powerful?
The lich king: The corrupted Arthas made into master of undeath, damming thousends of restless souls to his will.
Kil'jaeden: The lord of the legion, second most powerful ruler to it after Sergeras.
C'Thun: Old god of southern kalimdor.
Yog Saron: Old god of death.
Algalon the Observer: herald of the titans
Deathwing: Aspect of the earth, now aspect of death.
Illidan Stormrage: Brother of Malfurion and betrayer to all
Kel'Thuzad: Arch lich and ruler of cult of the dammed.
Kael'thas Sunstrider: Ruler of the blood elves.
(It seems more one sided for many, but wanted to include several by comparison)