Ruined XXX Monument Site is lowest level, Ruined XXX Crystal Quarry is highest level. Central XXX Sparking Transmitter is lowest level, Central XXX Data Mining Site is highest level.
The level, however, does not necessarily correspond with better loot but rather more difficulty to scan the signature and more high-grade containers to hack, which might have better loot inside … but usually they are the ones that end up empty in my empirical experience.