I would say Thor. Thor is said to be as powerful as the Hulk,
who is said to be as powerful as Superman. Power girl is less
powerful as Superman and at the same power range as Supergirl. Then
add the fact that Thor can control and hurl lightning bolts. In
Kingdom Come, Captain Marvel almost killed Superman by repeatingly
hitting him with lightning bolts. It could be said that Captain
Marvels lightning bolts were mystical, and magic harms Superman. It
all depends if the power of Asgardian lighting is magic related,
and it could be. AS Power Girl is less powerful than Superman, Thor
would mop up the place with her...in my opinion.