The first game that comes to mind for me is Civilizations 4. I’ve probably spent hundreds of hours playing but after getting used to 5 and 6 I have a really hard time going back. Going back and forth between 5 and 6 I need to rethink some strategies but with 4 I feel like I need to rethink everything. I don’t know if it’s because of the tech tree layout in combination with the civics system, things like unit stacking, or maybe just a bunch of little things but it takes me a while to readjust.
Space Empires V
It’s very old, unfinished and jank as fuck. The ai was never very good and could be steamrolled easily with the right tech tree. But those first few turns while exploring and setting up colonies without knowing exactly which tech your nearest rivals would have or if they were planning an invasion was always very fun. Then it would turn into a tedious logistics game of trying to move your fleets or decommission ships that took you the majority of the game to build.
Also, Space Rangers 2.
It’s like an amalgum arcady space shooter but somehow turnbased and space RPG text adventure. It was always very buggy with a UI that is ugly as hell.