The whole X3 series is excellent. One of the great space trading games. It shows its age in some ways (time acceleration doesn't last through a jump gate and graphics are not modern) but this is still a HUGE universe to explore as a fighter, a trader, or any role you choose. You can stay solo or build a huge fleet, with carriers, fighter squadrons and factory complexes pumping out all the missiles and shields you can't easily get elsewhere. Become a billionaire through selling narcotics, if that takes your fancy!