From what I've seen, there isn't too much rhyme nor reason to what NPC rats drop. Generally, they drop modules and ammo they would use on their ships, but not always (I've seen Guristas drop laser crystals, Blood Raiders drop ECM, etc.). Generally:
Blood Raiders:
- Laser Turrets/Crystals/mods
- Tracking Disruptors
- Energy Neuts/Vampires
- Armor mods: hardeners/plates/reppers/etc.
Guristas:
- ECM mods
- Hybrid Turrets/Ammo/mods
- Missile Launchers/Missiles
- Shield mods: extenders/hardeners/etc.
Angels:
- Projectile Turrets/Ammo/mods
- Target Painters
- Just about any type of shield and armor mods
Serpentis
- Hybrid Turrets/Ammo/mods
- Sensor Dampeners
- Armor mods
Sansha:
- Laser Turrets/Crystals/mods
- Shield mods
- Energy Neuts/Vampires
Mercs will drop just about anything, as their fleets are diverse. Drone ships only have drone alloys in their wrecks. EoM will mostly drop missile/hybrid equipment and armor mods.
Everybody drops drones and other general-use modules.
I don't have a whole lot of experience fighting Empire rats, but I believe they are similar to what would be found on the respective ships.