I won't attempt to redo the whole thing, just some guidelines:
More damage = higher on the list.
Harder to intercept = lower on the list. Hit what you CAN intercept first, you know?
The sooner you start shooting at it, the better your chances of intercepting it. This is especially true for torpedoes, where even a near hull interception will hurt a lot.
Obstructions should, IMO, be low. Fly around the asteroids, don't make your guns prioritize shooting them,. Target them manually if you want them gone.
This is not for player concerns alone - the AI has a horrible tendency of flying into these things on the main menu, and they can be as devastating as a torpedo to weak rear sections.
Guns that fire TONS OF BULLETS (i.e. repeating artillery) = lower on the list. You've probably got more important things yo fire at. On the other hand, one well placed Flak burst....
Repeating Arty is tame, in terms of bullet hell. Vulcan cannons and pulses - now those
are gonna be low on the list.