I loved Alpha Centauri when it came out. The terrain height system and being able to terraform the planet made it so much more interesting than Civilization (you could raise mountains to create more fertile land on one side and dryer land on the other, leading to some interesting strategic approaches, for example)
It's sad that id didn't receive more favorable reviews and maybe a successor or two
Played it for a long summer during the hours when it was too hot to go out. The aesthetics (terrain, music, videos, story) was beautifully dune-like and the ability to tailor units to your needs and playing style was fairly unique.
Edit: great review. I'm happy that AC hasn't been forgotten. Unfortunately there is no good way to play it on modern set-ups, I wouldn't even know where to download it from.
It's sad that id didn't receive more favorable reviews and maybe a successor or two