For Systems/Stations/Planets/Ships for Information/historical information and how they look. All these pages are mostly up to date.
Then the Commodity page(s) because you get a quick overview what you need for a POB and where you get it, what you can mine where and where to sell it or what is contraband. The latter is not always up to date and/or accurate though
NPC's Faction pages are outdated when it comes to the storyline and diplomacy, but i still use them just for the nostalgia.
I also use the Server commands, Restart Guide and ID pages from time to time. Mostly because of fast access.
What i don't use are the equipment pages, because the Wiki and its design has no real advantage over FLStat there. Pictures of a Gun or how the projectile looks is not that important to me for example. Don't know when i last visited one of these pages looking for information.
tl:dr:
Most frequently used: Systems, Stations, Planets, Ships and Commodity pages.
Sometimes in use: NPC's Faction, Server commands, Restart Guide and ID pages