Historians are all about context - measuring things against their own time periods, the judgments and opinions of contemporaries, the challenges that individuals faced within their own times. So we don't generally rate people on any kind of objective scale (there isn't any way to do this objectively, as there's no 'perfect' standard for a military leader against which to judge everyone else - a good military leader does well with the unique resources and challenges of his or her time, and only makes sense when viewed within this bigger picture).

Because we're trying to understand people within their own context, there does tend to be a lot of debate. This is because there are a lot of different factors to consider, and each historian will make a case for the value of his or her particular way of measuring a situation. This is usually a good thing, because differing perspectives are often complimentary and help us see more sides of a situation than you'd get by just ranking someone on an arbitrary and limited scale. Complexity is good, because the world is complex.

And historians are ultimately interested in much more than the great men of the past. We want to know how these men fit into their wider societies, how those societies shaped them and they shaped their societies, and what this meant for other less famous people who also participated in the past outside the spotlight. We want to understand the whole picture, not just the bits on top, and try to see how all the pieces work together to produce the exciting complexity of the past.