In order what i see here i would recommend:
Harbinger: probably the best but good luck running it on modern hardware(i heard it has alot of issues, but correct me if i'm wrong)
Space Siege: decent but overly simplyfied it is fun though if you can get past of it's simplicity (if you loved diablo 3 you will probably like this)
Restricted are: interesting game, a bit clunky but very interesting. The setting and atmospere is the most unique of all metioned in this post but it also take the most effort to get used to.
Greed: only buy if for some reason all of the above doesn't work for you: it's too simple, too short, incomplete,...It works but it's like a small snack or something to keep you busy while a better game is downloading or coming in the mail
This is a HUUUUUUUGE stretch but you might consider ZAX. You are an alien baddass shipwrecked on a nasty planet forced to basically dungeon hack your way through quests in order to fix your ship. A few cool things: a synthisizer (spell) that makes weapons and stuff for you and an interesting mouse control scheme similar to Abuse. Bad things: like any dungeon crawler it can get boring and the graphics while not horrid are not going to have you shitting your pants.
Here is a link. http://www.mobygames.com/game/zax-the-alien-hunter
Never heard of this one but it looks really interesting (anybody know if there is a way to get it digitally?)