Alpha centauri...damn, I installked that game a couple of months back and I deleted it just a month ago...it was cutting into my real life! That is a damn near perfect game...everything civ1 was, and more
Personally, I think 'screw the civ franchise'; let Sid create 'Milky Way' or something...starting at the industrial revolution, up through space flight and colonization so you can run multiple civ-worlds, each with it's own difficulties and bonuses.
And get the game to look like the citymap in civ3 throughout the game (like civ3 should have done...there was no excuse to have a plain flat map during the game: we should have been able to scroll through it like a globe at high magnification...like the city-view).
And for me: AC>civ>damn near any game considering the amount of time I spent on the first two

Civ2 was was just civ1 with slightly better gfx...and civ3 was just civ2, period.
Now all I need is for Alien Crossfire to work on winxp ('cause whilst Alpha Centauri does, installing Crossfire causes crashes to desktop

).