Both Darkstar One and Freelancer are good games.  Both are open-ended
space-sims with storylines.  In my opinion, Freelancer is probably the
better game.

You can fly several ships in Freelancer, while in Darkstar One you only fly
one ship, which you can heavily modify as the game progresses (must follow
storyline to gain access to new solar systems).  Another advantage to
Freelancer is the huge number of excellent mods which vastly expand the
original game.

I'm not familiar with Battle Engine Aquila or Lock on.