Well, if you really want to know, here is a spoiler that seems pretty obvious from the beginning:
Harry does ultimately defeat Voldemort.
But if you really want to know more than that (still no real specifics though), then here:
He really doesn't become that fantastic of a wizard throughout the whole of the series. He shows great potential, and learns certain things very well, and fumbles his way around pretty well when needed, but overall, I'd just say his ultimate victory is due to luck, good allies and
probably a bigger spoiler, if my memory serves.
the sacrifice his mom made for him which is talked about in the first book or so. He does hold his own against Voldemort, which is more than most are able to do, but is it his ability, or is it because of the protection his mom put on him? I'm still undecided.