Tough one. For me it would have to be Full frame, ergonomics, (do not like small body, or minimal grip), and maturity. My ergonomics requirements leaves two choices, Nikon Z series, will not mature until at least 3rd generation, and Panasonic S series. Ergonomics are excellent and Panasonic have the experience in mirror less so that gives the S series maturity out of the gate. Personally I would rather stay with Nikon, however if I were planning to switch to mirror less in the immediate future, (professionally, money being no object), I would have to go with the Panasonic Lumix S series and the L mount alliance. My opinion based on reviews.