To be honest, I think they'd all win given the circumstances. In the Vault, Atheon would win (his powers are restricted to VoG). In CE, Crota (same with Atheons powers) and in PoE Skolas would win since none of the other guys have powers, just their Torch cannons and Swords.
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Crota was in his weakest form and couldn't leave since we destroyed his soul. He was essentially and empty vessel. In any of his other forums no guardian stood a chance. Didn't he kill 1,000 or 10,000 guardians alone when they tried to retake the moon. There is already and existing thread/poll with hundreds of good responses and lore there.
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Crota can't be harmed without the Blade of Crota, though.
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However, Crota would win anywhere because with no swords to kill him, all you can do is stun him.