Circe is all over the place as far as general power levels go.
At her peak she's been able to effectively seal off the entire Greek Pantheon with little trouble and been shown as a peer of the Spectre, but at her general "I'm going to try and get Wonder Woman to admit hypocrisy by forcing her into situations where she has to kill to survive" levels she has issues.
So each character at their peak... Circe.
Each character based on standard showings... I would probably still lean toward Circe, but only marginally so. Her magic should be > His, but the hammer throws a wrench in her game.
If it's Wonder Circe (as pictured), then Circe for the solid, possibly 9/10.
"Regular" Circe? Without prep, I'd edge with Thor. If he can manage to get his hands on her (granted, not an easy thing), she's done for.
With prep? Circe. Not a stomp, necessarily, but around 6-7/10. She's worked with/against artifacts in the league of Mjolner before, and if there's anybody who could (with prep) finds ways to mitigate it, it's Circe...
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She *can* funnel her magic into physical strength and become near his level physically, but this requires spending less magic on actual magic, and of course she's not going to be as skilled in melee anyway (as she's found out when taking on Diana, who wins even when she's physically amped, though the magic bolts still make it dangerous).
Circe's is best at the range game.
Not easy, Circe can throw down major magic mojo and is goddess of magic. Circe and Loki are comparable in a lot of ways only where Lok forces on illusion, Cir focuses on transformation.
She could turn him into a frog or something fairly often.