I have been reading the upanishadic Bhashyas by Adi Shankaracharya, and once thing I noticed is, in some of them he gives rather terse explanations, as though the additional points/inner meaning, must be obvious to the readers, whereas in some others, he goes into extensive detail on bringing out to the light, the true philosophic tenets of the sruti. I think this will help us get some insights on the chronological order in which he could have written the upanishadic Bhashyas. In my assessment, he has gone into tremendous detail, first in the Brahadaranyaka, and then into the Chandogya. Probably the Kena can be included (due to the two levels of bhashya). Of course this is not including the Brahmasutras and the Gita. Among the upanishads, I would rate the Brahadaranyaka as the one which is the lucid-est if that's the term.