Probably Etrigan. Magus' best feat was transmuting an entire planet into zinc as Adam Warlock. The Thanos Imperative feat was likely The Church that did that.
I don't know how powerful any of those people are (except MM) and Phantom Stranger isn't exactly a fight/feat sensation, so it's hard to place him power wise.
I DO know that "standard" Etrigan knocked Superman into orbit with one punch. So he's a damn powerful "tank mage".
Regarding Magus, I know he's MUCH less powerful than Lord Mar-vell (because he was PWNED by him). I forgot what he did fight wise in Thanos Imperative and the run up to it.
If Stonehenge Etrigan is at least Trans level, then he got this.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
I'm almost 100% positive it was his Church's tech that brought the worlds to the Fault and detonated them : http://scans-daily.dreamwidth.org/1978536.html
That, the fact that Quasar and Nova were putting up a damn good fight against him, and Lord Mar-vell instantly pwning him leads me to believe that if Stonehenge Etrigan is at least Trans level, he got this.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.