Has to be Sid Meier's Alpha Centauri. It has held my attention for 12 solid years and when I play it I must do so for several hours. You can tell yourself one more turn, but that turn leads to another and hour could pass without you evfen knowing it. The classics are always better than you new "more graphics, less game" sort of games.
For me Command & Conquer Generals Zero Hour
I just won another skirmish match about an hour ago.
Robot Unicorn Attack.
You have been warned.
With the recent changes to WoW it has become addictive again, Blizzard have actually took time to update the game engine and the quests to be more enjoyable.
Hell even the horde get a Roller Coaster!
All times are (GMT -06:00) Central Time (US & Canada). Current time is 5:18:50 PM