My one is Sid Meier's Civilization - Specifically Civilization 4 - which is now a very old game, but I've never really got into the more recent releases. The game has so many nuances and problem-solving challenges involved in building a civilization, protecting it from enemies and researching enough science to still keep me hooked even today, despite the graphics doubtless being very dated by today's standards.
And If I'm allowed a free plug, I also help run a monthly game-of-the-month competition in which people across the World independently play a Civ4 game, all starting with the same map and see who can get the 'best' victory. Competition at
https://gotm.civfanatics.net/, and discussion forums about each game at
https://forums.civfanatics.com/forums/civ4-game-of-the-month.165/. I don't think it'd be hard to deduce which person I am on the discussion forums
)
A long time ago I used to be addicted to Chris Sawyer's Transport Tycoon (I guess not surprisingly for a rail forums site
), but eventually lost interest because, although I loved the theme of that game, I found the gameplay just too limiting. To be fair, it must have been cutting edge when it was first released back in the 1990s. Sawyer later released a follow-up, Locomotion, but to my disappointment it was basically the same game, with all the same gameplay limitations and just slightly better graphics - and no longer cutting edge.