As far as 2D space sims go, I would vote Space Rangers to be the best of all time, even better than Star Control 2.
Space Rangers offer a great variety of gameplay: combat, trading, piracy, exploration, salvaging, upgrading ship and skills, text adventures, planetary battles, arcade minigame. There is always something to do, and all playstyles blend nicely together for a complete and satisfying game experience.
This game has a living and breathing world, where every NPC has a purpose. Traders carry goods from one planet to another, pirates try to convince them to offload those goods to them, diplomats and transports fly between planets doing diplomat-y and transport-y stuff (gameplay-wise, they are either used by pirates to extort money, or fight the said pirates). There are also rangers, like yourself, who can be pirates, or traders, or warriors, and who behave accordingly. Each of these NPCs is persistent, which means they don't just disappear forever when you leave their star system. In fact, you can use the in-game search functionality to track any NPC across the galaxy, by simply typing its name (you could even play it as a stalking simulator).
I want to mention that Space Rangers have a very flexible game difficulty setting. On the lower difficulties, your enemies don't control many star systems, so you can freely fly across the galaxy, trading and completing government missions. On the highest difficulty, the enemies control most star systems, and you are left with half a dozen (or less) systems that you can visit without getting annihilated by a swarm of enemies within a minute. This adds a kind of "fighting a losing war" desperation feeling, when you must fight for the very survival of your species, which I personally find enjoyable.
If you are not averse to the idea of turn-based space battles occuring on a 2D plane, you should definitely give Space Rangers a try. And even if you are averse to 2D space battles, Space Rangers HD offer the option to play text adventures, planetary battles and arcade fights separately from the main game, so you can buy it just for that. Speaking of additional content, this game also includes a MP3 soundtrack (no DLC necessary) with 33 awesome space-themed tracks - you can find them in the "Soundtrack" folder in the game's installation directory.
P.S. "Space Rangers HD" is actually an updated version of "Space Rangers 2". If you, like me, prefer to play games and watch movies starting from the first part, there is also "Space Rangers 1", which you can probably find somewhere on the Internet (it's not on Steam, sadly). It's a bit rougher around the edges than the second part, but it still possesses the same qualities that make it a great game.