Hmm tough question: Xcom enemy unknown/within, dragon age origins/expansions, darkest dungeon. Those I’ve beaten and come back to multiple times and they are still fun.
Runners up include Pokémon lg, Zelda lttp, bg3, slay the spire, mass effect, civ, and stellaris
not stellaris but 4x with action point type system and card based actions. All in the stellaris universe/flavor. Sounds like fun to me