I'll be interested to see what is revealed in Superman #13 on Wednesday. If Clark bench-pressing the weight of the Earth is actually proved to be valid, then I'll have to go with Supergirl, for the Superman books editor, Matt Idleson, said that Supergirl is physically stronger than Clark himself, and I DON'T see Thor able to endure even one punch from then-proven world-moving figures as he's been written the past few months, especially in the recent Phoenix 5 series.
Superboy is a non-factor if SM13 pans out.
If Superman13 doesn't pan out, however, I'd have to give Thor the edge.
He has far more fight experience than either Kara or Connor, who are often slow and unsure of themselves in fights, and neither Kryptonian has demonstrated exceptional durability.
SM13 valid: Supergirl
SM13 hoax: Thor