"Primary Name" The site's "Primary Name" will give you a very rough idea of its difficulty. Don't take this as gospel, some "lower" level sites can be significantly more difficult than "higher" level sites depending on how you're running it, a difference in faction, or a difference in "Modifier" (explained in the section below).
Easy -> Hard (Anomaly) Hideaway -> Burrow -> Refuge -> Den -> Yard -> Rally Point -> Port -> Hub -> Haven -> Sanctum
Easy -> Hard (Signature) Hideout -> Lookout -> Watch -> Vigil -> Provisional Outpost -> Outpost -> Minor Annex -> Annex -> Base -> Fortress -> Military Complex -> Provincial HQ -> Fleet Staging Point