Ultima and Wing Commander, I know I’m showing my age but I guess it’s the nostalgia. I still go back and play through them.

Civilization 4: Fall from heaven: Ashes of Erebus. It can be played multiplayer of course, but I almost exclusively play single-player. The lore is fun, the event system can be interesting, the combinations of awesome Civs+Leaders+Religions+Magic really up the replayability. The end game with hundreds of units amongst a large number of factions with hell terrain encroaching makes for a very dynamic endgame.