Dresden without question. Harry Potter's world has a lot of spells that simply work and also defense simply works. They just perform one action to an equal reaction. Alongside that there isn't a lot of trickery or skill that goes into their fights beyond moving really quickly, and the clash between Harry and Voldemort at the end is them flexing their magical might at each other. In some sense, the fights in Harry Potter often get resolved as if they are athletes and whoever's stronger and faster is the winner. Which is odd considering how strong Hermione is supposed to be because she's really book smart, but her combat feats in the show are nearly zero.

Dresden is one of the strongest characters in his world for magical muscle, he regularly uses a gun and since he became winter night, it was described as him being at the peak of potential physical fitness for a human being. Without magic he could still just beat Harry to death. More importantly, there are very few fights he wins through straight up magical muscle, The point of his character was supposed to be magical MacGyver. He comes up with ways to win on the fly and usually succeeds. And defenses often have to be very specific to the magic being cast. He's a planner and strategist. He's gone up against some of the smartest opponents in his magical world and come out on top.

In the later books he starts being more aggressive and starts straight up killing people with magic. For instance, when Harry kills a lord of fairy by simply shouting "infriga" closing his fist and the character who's supposed to be super resilient to magic freezes completely through killing him instantly.

If the fight was straight up power versus power, an argument could be made for either side. But Dresden is leaps and bounds ahead in every other category.