Railroad Tycoon
The sequels never really did it for me. Transport Tycoon sucked up the hours, but nothing really did it as much as the feel of that first game. Maybe it was the time — the CGA monitor (which couldn’t cope with Civilisation), no mouse of course. I did play it years later and it want the same.
I doubt that feeling can be recaptured. It probably never existed - its a sepia tomes memory of a feeling from a different time with different expectations - but for some reasons the original Railroad Tycoon really stuck with me.