Science fiction wargames
I am not generally a missive fan of science fiction wargames, normally happier dabbling in historical games, but a few SF games do stand out as worthy of repeat play.
Traveller by GDW
Like Dungeons and Dragons...but with more thinking.
For further details see wikipedia
Full Thrust
A fun miniatures game. Anything with vector movement is worth playing (see Triplanetary above!).
For further details see www.boardgamegeek.com
For an example of a game see the Full Thrust page.