Civilization IV <- Should be removed, rules state you can't have 2 games from same franchise
Heroes III of Might and Magic
Master of Magic
Command & Conquer
Crusader Kings 2
First game that came to mind was Starcraft 2, but I'm confident someone else well add it or the original Starcraft. Then I considered Rise of Nations but thought going through the ages I could get through Civ even though they play vastly different. So I ended up with Crusader Kings 2 because there isn't really anything quite like it. I've poured hundreds of hours into this game and gladly give Paradox more money for every major DLC that is released so they continue to support it. It has a huge learning curve but I think that adds to the satisfaction you get when you are eventually able to bend people to your will.