If I had to choose one camera, period, then I'd choose Sony A7.
But if we're talking film cameras, maybe my prewar Zeiss Ikonta 521 with uncoated Tessar, or my newly-acquired Alpa 5. Not because they are the most efficient picture-taking machines, but because of their personalities: I like how the Ikonta's shell is just barely large enough to wrap around a pair of 120 film spools and is about as small as a 6x4.5 camera can be, while the Alpa has a certain unconventional logic to it, such as a mirror which swings out of the way via finger pressure - not super-speedy, but there's no mirror slap either.