Going with Paladin just because the magnitude of the power they channel seems to be limited only by the will of the Light. Mages are strong but need great magical artifacts to channel great power (like Jaina trying to drown Orgrimmar), and Locks and DKs and Demon Hunters are strong but all risk getting turned on by the powers they harness. When Tyrion shattered Frostmourne I was just like daaaaaaaaaaaaamn. Priest would be my second pick for similar reasons, and tying with Monks (thinking of Shaohao becoming one with the land), but I've never seen anyone command offensive and defensive powers like a Pally.
Though I imagine its something more like Rock, Paper, Scissors rather than zero-sum absolute power scaling. Like yeah, Arthas rolled Illidan but Demon Hunters don't train to fight undead really, they specialize in demons.