Kithara and I were just talking about this the other day.
Terms like Plump, Chubby, BBW, Full-figure or rounded are tossed around and even assigned to the same person. Because of this it is difficult to even come up with a ranking let alone a top of the list group.
I especially like Palomine's rough description from several posts above. Also, Feral's more recent attempt to delineate some bounds to the definition of plump moves in a good direction for it handles the relative differences of some of the 'contestants'.
A good example of this that others have mentioned is Nadine. Not only did she appear to sit right at some toggle point in people's minds but she has changed over time (matured, grown into her body, whatever) and now is soundly in this "plumper" range.
I am reminded of how astronomers rank stars. They have set criteria that help define relative differences and then categorize the resulting clusters. They do not first start with a label and then find ones to fit it. So, I may spend some time thinking about this one.