Civ4
MOO2
Civ2
Civ4 - added graphics, and alot of variety that made that game excellent. Promotions, Espionage, Religion, etc.
MOO2 - added customizable races. I could decide from the beginning, what specifically, My new Empire would be best at.
Civ2 - I created my first units here, literally. I felt an "Archer guard tower" was missing, so, I added it. I felt "Arrows" similar to the cruise missle style could have been added, so, I did. Great flexibility for its time.