It's worth noting that Harry has signifigantly "leveled up" over the course of the books, so power level is a little more of a fuzzy topic than it might be. If we're goign to do things in pure terms, there is a decent argument to be made that Michael is actually more "powerful" than Harry (albeit less flexible) and before you scream, I'm just going to say: killed a _dragon_.
Of course that just further illustrates the fallacy of any purely power based model. Michael, as an example, has no fear of any foe, so long as his cause is just. He was willing to throw down with Mab, on her turf, with her pack of minions on hand. Some of that is "God on my side" confidence, but it's not all bravado.
Harry doesn't have that same raw capability, but for Harry, the cause doesn't need to be just, and the means can be much more roundabout. Even if Michael can win _every good fight_, can you really say, except perhaps on a case-by-case basis, who is more powerful?
-Rob D.
PS - Note, I'm leaving interactions with the Denarians out of the equation for a simple reason - the Knight's relationship with the Denarians is more complicated than it is with a random evil thing, no matter how monstrous.