Civilization or SimCity

[A Black Falcon]

I don't think there are demos of any of the three Civ games, but there are demos of Alpha Centauri and its expansion if you want to find them... (remember, Alpha Centauri is Civ, Futuristic Version) it gives a good taste of Civ-style gameplay. Just, obviously, different because of its futuristic look... which is more than just a skin. It really affects everything. Oh, and AC, unlike Civ, has real rolling terrain (Civ just has block terrain -- Plain, Grassland, Hill, Mountain, River...), and unit design (you get parts and can put them together to form a wide variety of vehicles), unlike Civ. And AC also was the first game in the series to have the nation borders drawn on the map. :)

Oh, and it was Brian Reynolds' last game at Firaxis...