13 mini. I know it's been discontinued, but you might still be able to find one for sale in some store. The battery life is improved over the 12 mini (which had a bit lackluster battery life), the camera should be quite decent, and it's plenty fast and fluid. And it slots in size between the 5s and the 8.
Honestly, if I had the money, I would've bought one in May, but I settled for a used 11 Pro, which is still good. Actually, it probably suits my needs even better because of the added bonus of a telephoto camera and better battery life (now that I actually had the battery replaced, lol), and the size is still reasonably small, but I still need to take it back to the repair shop that replaced my battery to see what's wrong with the proximity sensor. Problems of buying used stuff.
EDIT: now I see eizbaer was faster, but yeah, I'd definitely recommend the 13 mini over the 12 mini, the battery life is better, and it should be easier to find one.