PopTop. Railroad Tycoon 2&3 and Tropico 1&2. They got bought by 2K, which eventually killed the studio. The Railroad Tycoon series is dead. Tropico is still around, but I'm not excited about the latest interation. Some of the guys tried to kickstart a new Railroad Tycoon but it didn't fund. Phil Steinmeyer was an underrated developer, though I believe he's retired today.
It's too bad it worked out that way. I think they could have been on the level with Paradox as far as strategy games are concerned, but focusing more on economic games, city builders, and the like. On Steinmeyer's blog he said he didn't think there was demand for heavier games anymore about mid 00s. That might have been true then, but so many games out now prove that wrong.
I just looked at my Steam stats today and 52% is RT3. There may be plenty of games that surpass it in visuals, but game mechanics are a different story, and without a stock market I don't see much of a point.
I played Tropico 3-5, but gave up after they refused to give us highways and overpasses.