Some of you might be familar with Doug DeMuro, who does a weekly review of an interesting car on youtube. He can be quite a polarizing figure, but I generally find his reviews quite interesting.
He recently did a W126, and while he gets a few things wrong, its quite a good take on the W126, and those of you who are not familiar with some of the differences in the USA models can see them clearly in this video.
He also ranked the S-Classes in his opinion from the W116-W222. His rankings were as follows:
I disagree with him putting the W140 at the top, I would put it close to the bottom myself. I find it over complex, and in my view its on of those mercedes models that are the awkward transition between one design language and another. The same can be said for the W114/5, W210 and a few others.
My list would be (excluding the 221 and 222 as I have no experience with those).
From a styling point of view, I would put the W221 in at 3rd behind the W116 and the W222 behind it.