I've been thinking a lot about necks lately because I've been working on my left hand, including giving the thumb the attention it deserves and never really got from me.
Like most people, I tend to find the familiar more comfortable, sometimes more so than something that is better for me physically. I used to prefer thin "C" shaped necks, possibly because I spent more time playing steel string guitars, and I felt that chunky classical necks got in my way and/or my hand was too small (I do have smaller than average hands). Today, though, whether because I'm more focused on classical, and taking it more seriously, have better classical LH technique, older hands, or all of the above, I prefer a slightly thicker and more "D" shaped neck.
These days, my main concern with necks is effortless placement and leverage for the thumb. Given good hand posture, if I have to use any muscle tension to put thumb on neck at the best spot, then the neck isn't working for me. I've also learned that proper thumb position varies a great deal more than I'd thought previously, which makes me more aware of how the "right" neck size and shape can facilitate effortless leverage in different situations.
That doesn't necessarily mean that only one exact size and shape will work for my hand, though. More likely, a range of size and shape combinations can give me what I want. But it probably does mean that I'd have to try a few necks, with an open mind, to find a "right" one.