For those who listed Alpha Centauri, are you referring to Sid Meier's Alpha Centauri, or the Brian Cranford one?
(If anyone not related to me even KNOWS what I'm talking about, I'd be willing to give them money.)
Most addicting games I've played, in rough order:
WoW
Diablo II
Dominions X (where X = 2+)
FF Tactics
MULE (and many other classics)
Star Control II must be named...
I'm sure I could come up with others...