Culling drakes- selection

I'm curious to find out what all you duck people out there are selecting for when culling drakes. I have been looking for aggression mostly, as well as size, personality, nervousness, and friendliness towards humans. My small flock is such a cohesive little unit that none really seem to stand out in any way, except one who appears to be the lowest of the order and often gets shunned by the group. What are you looking for in your drakes?