Freelancer is my go-to space game. Or at least was, until ED came out... Not sure I'll ever be able to play it again once the full version of ED is released. But some things I like in Freelancer I probably won't find in ED. I really like the radio chatter, and the fact that you can hail any NPC. They all have an origin, a destination and a mission. This just makes the game world feel alive. Also the simple but well made way that you can walk out of your ship and go into a bar in space, from where you can see the actual space outside from a large window. That's awesome.
I also kinda like
X Rebirth. It's not as bad as many seem to think. However, the interface, controls, and general polish are indeed awful. But I really like that you can land on stations and walk out of your ship, even if its so poorly made. When ED finally lets us do that, it will be...
When I was a kid I loved
Rogue Squadron even if it's not actually a space sim. It's worth mentioning, as it was the first flying game I played, and the first one I used a Joystick on.
Another game that is not technically a space sim, but contains a lot of the things I love about the genre, is
Spore. While very limited in terms of gameplay, this game is amazing in so many ways. The scope is mind-boggling. I just love how you grow from a simple cell in the ocean, to a space civilisation spanning dozens of systems. I love that you can land (albeit as a hologram) on any planet, walk on it and approach the fauna. Or find a dead rock, and inject life into it until you make it your perfect little paradise with your favorite plants and aliens, and then freely walk on it, before you jump back your ship and fly to the other end of the universe. I'm pretty sure no other game will ever let me do that again, so I cherish it for that. Also, the Galactic Adventures expansion was awesome, I'm still working on some missions I build from time to time, even if I'm probably the only one who will ever play them!
I always loved the concept of space games, being able to explore the universe, see a tiny dot in the background of space and know that you can actually fly to it, land on it, and it's a whole world full of people. But in practise, apart from the aforementioned games which gave me that in their own way, I never really found the perfect game for me. They are usually to complicated, or not immersive, or have an awful gameplay... ED is really going to be the game in which I can spend as many hours as I want, really being a space pilot. Star Citizen will probably give me that opportunity too, in a different way.