Great thought provoking question. Here's my criteria:
1. A camera I can look through (not down into the top of or at an LCD)
2. The viewfinder and how focus "Pops" for my eyes
3. The size and weight of the camera and how it feels in my hand
4. How my shutter finger interfaces
5. My history with the camera
6. The industrial design of the camera
Examples, my M3 with Tom's mini-Softie hits 1,2,3,4,6 and is one of my favorite cameras. Am still developing a history with it. My Canon F-1n, where I've installed a bright matte screen and custom diopter hits 1,2,3,5 and is probably my second favorite shooter as we have a tremendous history together. A very close third is my Nikon D4 which hits 1,2,4,6 though it's quite heavy and we don't yet have much history.
A camera that I dreamed of owning for many years was the Rolleiflex 2.8. The industrial design is stunning and it has been involved in making so many historic images that I love. But once I had one, I found it very hard for me to focus, images didn't pop, I hated looking down at the top of the camera instead of through it, the image moved in the opposite direction than what I was used to, and my hand and shutter finger just didn't fit it at all, so it got sold. I needed a MF camera though, so I found a Hassy, and with the addition of a prism finder, I was able to make it work as I can now look "through" the camera and the focus pops.
Guess the question provoked quite a number of thoughts. 🙄
Best,
-Tim