I voted "other".
First off, I kind of disqualified Sid Meier's Alpha Centauri / Alien Crossfire -- it's SF themed, but not truly a
Space 4X game per se -- or it would have won out, hands down.
...
So instead, IMO it's currently a tie between SE4 (v1.49 only so far, ordering SE4/Gold soon!) ... and MOO2. Stars! comes in a close second to that tie.
I won't vote clearly for SE4 just yet, since it's (for me) still got hte blush of newness on it, and I
mgiht tire of the sheer
length of an SE4 game in the next year or so.
MOO2 (barring current video card conflicts) regularly returns to my HD for a month or three at a time.
SMAC/SMAX (the SF-but-not-space game I mentioned above) makes repeated visits to my HD, sometimes staying resident for a year or more.
Hopefully, SE4 will do likewise, having a space essentially reserved for it on the HD near-perpetually.