Looking for a Space Sim

I've never played a space sim before and I'm getting very curious about it, so could you recommend me a good space simulator game? To be more specific I'm looking for something that's good for getting into the genre. By that I don't mean I want something simple, I heard that many of these games can get very complex, which is fine, it's part of the charm I suppose. What I'm trying to say is recommend me something that represents the genre very well, let it be about complicated economic system, exploration or space battles, whatever is there to it.

It doesn't have to be on steam and it can be an older game too.

Thanks in advance!