My understanding is that he is normally the most powerful Celestial. The only one that might be higher is Scathan, but I'm not sure if he's canon, since he only showed up once and was a very stupid character.
As far as who beats him, I really don't know. I would assume he is pretty well above Galactus' normal power levels. Don't know about Phoenix. Also, while Death is the sister of Eternity, she's really never been portrayed as equal to him in power, as far as I'm aware. One Above All was one of the two Celestials who engaged Thanos during the Infinity Gauntlet event, although that doesn't mean much, since he was beaten about as easily as everyone else (everyone who wasn't Eternity, anyway).
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