although I am biased towards the early 1-13 run, I think the interior art was at it's Kirby best around issues 39-73 range...highlighted by the DD/Dr Doom issues (39-40), SS/Galactus Trilogy (48-50), all the silver/doom/black panther issues, and finally the Blastaar/sandman splash panels in issues 61-63 were pretty good. My two cents.
I like this post too and agree with much of it. That said, I just looked over the scans for many of my covers and find it hard to be more specific than #'s 35 through 95. They're all so good!