Haven't played the new ones but I used to waste hours and hours in X3. I remember the mods back then were huge, with all kinds of features (mobile mining, hidden wrecks, custom HQ bases, hiring NPC traders / NPC fleet ops ...)
The only thing I missed was the multiplayer... You had this huge galaxy that felt alive but you were always alone in it. Freelancer/Disco on the other hand has the people but you never get the feeling of huge, living galaxy... more like a tiny pocket universe (the tiny planets only add to that feeling )
)