One knife, and one knife only, from now until the end of days, I'd have to consider all factors and choose a knife that does all things well enough, rather then a knife that excels at one thing, but lacks greatly at others. Considering also, if i am to even entertain this notion, that if it would so ever come down to such a horrific choice, S has clearly H the F! And we're living in troubled times, and would likely choose my bk5.
Tough enough I'd be hard pressed to break it
Simple enough to be field sharpened with ease to a very fine edge, and handles are easy enough to replace if they would become damaged.
Fixed blade doesn't require as much effort or worry over cleaning and maintaining (potentially replacing) the extra moving parts.
Large enough to chop, profiled well enough to slice, pointy enough to stab.
I'd rather have more knife and not need it, then limit myself in size, and potentially find myself in a situation wanting, especially if it's the only knife i'll ever carry again.
If I was allowed one fixed and one folder, I'd add a SAK huntsman or s17 evogrip to its front pouch for its simple versatility.