Hmm.....I've always loved X3 series from Egosoft, of which the most recent is X3-Terran Conflict.

It is a space simulator / sandbox game so to speak. Basically, you control a space ship as expected in a huge open world with I think ~200 sectors, 6 main races, hundreds of ships, etc...

There are story lines but they're optional and are in no way required for the enjoyment of the game.

The best thing of the game, though, is that you have an immense amount of freedom. You can, as one would expect, just build up a huge fleet of ships (oh, forgot to mention, you pilot one ship yourself, but you can control hundreds of other ships as well, allowing for fleet on fleet action) and just go picking a fight with a faction or invading a sector. Alternatively, you can also go a big economic route.

There is an extremely intricate economy present with a ton of goods that you can buy or sell. Everything that you buy, though, must be made at a factory that is supplied with the required materials. Prices are dependent on supply and demand allowing you to make a ton of money just trading and finding the good routes. You can set up automated sector traders that will gain in experience and become universal traders once you are richer. Once you have enough capital, you can build large factory complexes and just wait for AI traders to come buy your goods. In designing your factory complexes, there is actually a ton of research/math involved to find the perfect sector, sufficient asteroids for raw materials, the necessary number of factories to become self sufficient, etc...

I will be frank, though, and admit that this game as a horrendously large learning curve. You can easily spend your first 20-30 hours just kinda fooling around, not totally knowing what you are doing but once you get to know the game, it becomes deeply satisfying.