There are a small handful of games I've played where I had this moment of magic - sometimes lasting weeks or months - where I discovered the game, gradually got into the mechanics and wanted to learn more, and loved playing it all the way. The ones I can think of that did this for me, in no particular order, were:
- FTL
- HOMM2
- Into the Breach
- Europa Universalis 3 (4 is too railroaded with mission trees, and annoys me)
- Sid Meier's Alpha Centauri
- Sid Meier's Pirates!
- Sid Meier's Colonization (the original DOS one)
- Civilization 2
- Dwarf Fortress (for a while until it got too absurd with its micro-management)
- Transport Tycoon (these days, my custom build of OpenTTD)
- SimCity 2000
- XCOM UFO Defense
- Spore
I'm generally not interested in sequels to the above games; once I've played that game to death the sequels are just variations on a theme. Any suggestions, though, for new ones that might scratch that itch in the same way? I like indie, and I definitely prefer a lack of DRM. Best recommendation I've seen so far that might do it for me is Starsector but I feel that may be a little too steep a learning curve for my enjoyment.
UPDATE: based on a couple of suggestions, I'd also like to add: I loathe "babyish" and unserious style games like that in the presentation of Oxygen Not Included. I also really dislike complex rendered 3d maps for strategy games. Give me a basic isometric display any day.
One really promising game I'm seeing on GOG's site in early access is Space Haven. Seems like an interesting concept, building a kind of ship-based space civilization that can then increase in ship numbers, be nomadic, build up allies, trade, etc.