Juggernaut gets his power from the devil-god of destruction, one of the eight most powerful mystic entities associated with Earth: Cytorakk. Cytorakk is probably more powerful than Odin, but he is not interested in anything other than worship.
Cain Marko is not a very good Juggernaut, because he is wilful and not dedicated to the cause of Cytorakk. He frequently has moral quandaries. In practice he is weak at those times, weaker than Thing and Colossus.
But if he does not waver and if Cytorakk mainlines him with power, he is stronger than the Hulk and Thor.