Combat Commander, Empire of the Sun, or Enemy Action Ardennes?
Combat Commander:Europe if you want a fun 2 player game where each game tells a different story. The fun factor is very high on this one in my opinion.
Enemy Action Ardennes if you want an immersive solo game (well, 2 actually) which does take quite some time to complete.
There is a 2 player game included also of course, and I suspect that is a lot of fun *but* takes a lot of time both the play and learn.
Empire of the Sun I would not get. Especially not for solo play (one of the worst gaming experiences of my life).