Whenever I go to fire up some old games the first two, and the two I spend most of my time on are-
Ultimate Domain: a little slow but has an great city building and worker placement system and one of the coolest tech trees I’ve ever seen.
Space Bucks: also a little slow but is essentially a space sci-fi alien commodities shipping simulator. Start on a planet, win bids to ship to and from other planets, buy low and sell high, and improve the planets ability to produce while also improving your space fleet, optimizing for speed, cargo, offensive and defensive abilities.