Deserted Island Camera?
(you know, if there was a photolab or darkroom on said deserted island, and whole crates of velvia washing up on the shore at regular intervals)

Probably depends on what you shoot and already have.
For me, probably my Bessa and 21mm Skopar (either my Bessa L or R3A, either way i'd use the external finder). If I could buy something before I went to said island, probably same lens on R4A.
It's small, portable, pocketable, unobtrusive, great for hiking and produces great shots. (and has the uncanny knack that whenever I take only it out for a day of hiking and leave the SLR at home, there's some cool bird to shoot and all i've got is 21mm). Still, I can live without birds but I can't live without landscapes.
Doesn't do too badly for street and architecture with a slow B+W film and orange filter either...

Or maybe I'd upgrade to a GF690 or something for bigger frames with more detail, but I can't afford them...