Work ethic hands down.
As a supervisor - this single characteristics put a person a notch above the rest. People with a piss poor attitude generally don't have great ethic, so that doesn't really matter. (and if they do have great ethic/bad attitude - it's just as easy to make them go be efficient somewhere else where they can't bring others down)
Next step up - self-motivation.
The ones who have a goal, who don't want to sit around doing what they're doing their entire life...those are the ones that you can always count on.