I've owned the 8, 8 Plus, and X.
I'd say none of the above, for now. There are trade-offs with every one of them.
Best size: 8. Tradeoffs: Screen too small for some things.
Best camera: X because of OIS on both lenses (disappointing that 8 Plus lacks OIS on telefoto). Tradeoffs: Price. Screen is longer, too small. Other issues you've noted.
Best readability: 8 Plus. Tradeoffs: too darn heavy. No OIS on telefoto lens.
8 vs 8 Plus camera: Interestingly exposure sometimes seemed a tad better with the 8 over the 8 Plus when taking same image in same lighting conditions.
Someone earlier said she was quite pleased with the 8 Plus as a travel camera. This just goes to show that one person's "quite pleased" is another's "quite unhappy." All depends what your expectations and experience level are. For someone coming from a digital point-and-shoot who views pics mainly on the phone, Facebook, etc., the camera will be fine. For someone coming from a high-end SLR or DSLR, eh, the 8 Plus is still a toyish camera crammed into a phone.