The danger posed by space debris depends on the debris' mass, ballistic coefficient and orbit.
For a given mass of debris, a single large mass (like an intact dead satellite) poses a much smaller risk to other spacecraft than the same mass broken into fragments. However, massive debris stays in orbit longer, before it spontaneously deorbits, than the cloud of small debris. And the longer the large mass stays in orbit, the more likely it is to hit something and turn into a cloud of small debris itself.
Debris in orbits less than 600km tend to deorbit spontaneously.
Deorbit fuel cost is proportional to debris mass. But rendezvous fuel cost is proportional to the number of fragments that make up that mass.
Vulnerable target density varies widely with orbital altitude and inclination. It makes sense to make a priority of debris in orbits of high target density.
On the other hand, debris which shares a coplanar orbit with its targets has a lower relative velocity so it is less likely to be broken up by collision with other debris and less likely to do damage if it collides with a target.
Deorbiting uncooperative debris will be very expensive, so efforts should be focused on low cost: benefit debris. But which debris?
What combination of mass, altitude and orbital inclination make for the best deorbit targets in terms of damage reduction ?