Bmach, I realize what you are looking for. I've thru-hiked the AT twice and led quite a number of group hikes for one of the maintaining clubs. The short answer to your question is, "all three--frequently more or less at the same time."
A more useful answer takes a little longer. As far as I know, there is no one who would try to "rate" all parts of the trail, and, as Mouse has correctly noted, there is considerable resistance to this. Individual clubs usually do rate specific hikes, however. But these ratings include factors such as simple distance and driving time (do you have to keep a 3-mile-an-hour pace for five hours in order to make it home the same night?). Other factors include (but are not limited to) expected weather, treadway condition, stream crossings, distance between potential "bail-out" points, experience level with which the leader is willing to cope, and most importantly, amount of climbing.
Some of this you can figure out for yourself. Go to you nearest outfitting store and look for the maps and guidebooks for the areas of the trail that are within reasonable driving distance of your home. If these are not available locally, go to
www.appalachiantrail.org and order them. On the back of most of the maps there is a scale of altitude change so you can see very quickly what you are going to be dealing with. Often, there are comments on the trail on the map. You can also gain more detailed information on the trail itself from the guide book section.
Find a section that is say five to ten miles between roads (where you can "spot" a car) that appear to be fairly level and go day-hike it. Do that a couple of times and you will be well prepared to estimate the level of difficulty. Figure about two miles an hour when you are starting, spot the potential bail-out points, and leave yourself a couple of hours of leeway in your plans.
Happy trails, jcp