Sim City 4 is the best version of the Sim City games, and is 75% off on GOG right now, $5 / £4.

Cities Skylines 1 is the best modern city builder, 3D and a lot of fun plus well designed. But only really worth it when it’s on sale; lots of DLC and overpriced as a package when not on sale. Avoid Cities Skylines 2 - it’s just not fun and hasn’t been fixed - maybe they will one day fix but I doubt it 2.5 years in..