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.
I’ve recently started using the archipelago randomizer mod to get another taste of replaying it, but unfortunately the satisfaction of completing a rando pales in comparison to that first experience.
I will never not recommend this game. It’s so good and I can’t wait to see what Mobius comes up with next.