'Perimiter' sites are "easy"
'Frontier' sites are "medium"
'Core' are sites are "hard"

Gravimetric -> Hidden Asteroid Belts
Info: There may even be a chance that Faction NPCs show up and drop Faction Items/Implants

Magnetrometric -> Profession Site, mostly ArcSal (Archaology & Salvaging)
Loot: T2 Rig BPCs, Datacores, common & rare Salvaged Materials

LADAR -> Complexes & Gas Clouds (There are also reports about RADAR Gas Clouds, so they don't seem to have a fixed sig. type)
Complexes most of the time escalate when you shoot the Structures in the second Stage

RADAR -> Profession Site, mostly Hacking
Loot: Enc. Skillbooks, Decryptors, Interface BPCs & Components

"unknown space" could lead to a class 1, 2 or 3 Wspace

Guristas - Kinetic
Serpentis - Thermal
Mordus - Thermal & Kinetic
Blood - EM & Thermal
Sansha - EM & Thermal
Angel - Explosive
Rogue Drones - EM