In Order of Difficulty -Easiest to Hardest Site-
Hideaway / Cluster Hidden > Forsaken > Forlorn
Burrow / Collection
Refuge / Assembly
Den / Gathering
Yard / Surveillance
Rally / Menagerie
Port / Herd
Hub > War Supply Complex / Squad
Haven / Patrol
Sanctum / Horde

NPC damage type weakness:
Guristas - Kinetic
Serpentis - Thermal
Mordus - Thermal & Kinetic
Blood - Thermal
Sansha - Thermal
Angel - Explosive
Mercenary - Thermal
Rogue Drones - EM

NPC damage type usage:
Amarr -Thermal- EM *
Guristas - Kinetic & Thermal
Serpentis - Kinetic & Thermal
Mordus - Kinetic & Thermal
Blood - EM & Thermal
Sansha - EM & Thermal
Angel - Kinetic, EM, Thermal & Explosive
Mercenary - Kinetic, EM, Thermal & Explosive
Rogue Drones - Kinetic, EM, Thermal & Explosive