I haven't forgotten you asked, but I think I need to dredge my mind a bit... Remind me, later?
I should explain that for the most part Cliff, like most writers, seemed to like best the story he was currently working on...understandable.
But as far as specifically commenting on stories (of his) that he liked or did not like -- he did that seldom, at least in my presence; but when he did, it was often a remark about one of his shorter stories -- novels were not often commented on in that fashion...
So if I mention a novel or two that I thought he particularly liked, keep in mind that I'm doing so on the basis of remarks he made in other contexts.
That said, I can tell you that I think he particularly liked
Ring Around the Sun and
Time and Again.
I say that about
Ring because it kept coming up in comments on his work in general. And I saw that about
Time and Again because of the fact that the climax is set on what appears to be his own old family farm -- along with the fact that he tucked a character who appeared to be a lot like himself into the story.
I've thought, at times, that it may be significant that
Ring was in some ways his first novel --
CITY isn't really a novel at all, and
Cosmic Engineers was very out of character for Cliff in that period of his life -- he hadn't written at that length before, and he did it on that occasion only because John W. Campbell pushed him for something long enough to be serialized. I don't think Cliff really wanted to do it, but he tried to oblige John -- and in later years Cliff would consider that book a failure, in some ways: you see, he didn't like doing the kind of story we now think of as old-fashioned space opera, the ray-guns and rocket ships stories; he wanted to give John a story that (like the short stories he'd been selling to John for a couple of years at that point) relied on ordinary people rather than brainy scientists and heroic rocket jockeys -- but, as he would say years later, sometimes "you had to be grandiose in spite of yourself."
As for
All Flesh is Grass (which is how we got into this discussion): I think Cliff liked it, yes; but why is a little difficult to say... The book combined a number of themes that were among his favorites, but I don't think that by itself makes a book among one of his favorites. So I guess the reason for my judgment is simply that Cliff commented to me, at one time, that he had been able to put into the book a little of his knowledge about making a career in writing fiction...I interpreted that (perhaps wrongly, who can say?) as indicating at least one thing he was happy to have done.
I'll mumble to myself for a while and see if I come up with anything else about the novels...