It's kinda hard to narrow it down to 1 specific thing. I guess if you have to the poll so far seems about right. I would say personally that if you didn't have the right combination of the first 4 tho, that the question would be moot.
Like, let's say you have a fixed blade. But it's made out of crappy steel. Or fixed blade but it's only 3" and made outa crappy steel and 1/8" thick. In those cases I'd rather have a quality folder.
Out of everything tho, I'd say ease of sharpening would be the least important.