1. Civilization (I, II, V)
2. Master of Magic
3. Master of Orion II
4. XCOM: UFO Defense (too early to tell if the new one will make the list)
(Crap, I filled the first 4 with Microprose/Firaxis strategy games, guess that leaves Alpha Centauri in the dust)
5. Elder Scrolls (IV, V and anyway)
Honorable mentions to Mass Effect (I, II), Final Fantasy (VII, X), Tekken (3, 5, 6), Soul Calibur (I, II), Super Mario Bros. 3.