Rankings are pretty useless in differentiating between top-tier schools. If you'd like, you can check out USNews (
http://grad-schools.usnews.rankingsandreviews.com/grad/pub/search) and see what they say (although obviously London will not be represented, it being US news and all), but those rankings are essentially meaningless. Have a little fun with them if you'd like, or use it to narrow a list of 20 potential applications down to 15, but don't take it too seriously, no one in the field does.
That being said, the scuttlebutt is that Harvard tends to be more focused on domestic issues, and Hopkins more global health oriented, but you can find all types of public health research at both places, including many, many others. For example, UWash has the benefit of massive amounts of Gates funding and I always hear great things about UNC.
Edit: Oops, I see you were asking about the international schools specifically. Sorry I can't be more helpful. But I would point out that rankings are even less meaningful when compared across boundaries. Your best bet would be to find a student from the school.
