Not for everyone, but if it's for you Crusader Kings 2 is the end of the strategy rainbow.
Crusader Kings 2 is a dynasty simulator, maybe the best and only of it's kind. If you are one of the people who this game is made for, you will find a nearly perfect game with pretty much infinite replayability. This game is timeless.
It certainly requires more thought and intelligence than most games, and the learning curve is steep, but once you get your bearings, learning and playing the game becomes fun and easy.
This game boggles the mind with the depth of the simulation, and the fact that that no matter how much you play it, there is always something new and thrilling to discover or new angle or strategy to uncover.
My only criticism of the game is the combat, which has always been the weakest part of this series. It seems to have some depth, with the different types of troops, but after many hours of playing and winning I have yet to find a need for it - it is a numbers game, with combat automated, not very far from Civ I.
This game with deeper combat, maybe even the option to enter into Total War or Mount & Blade style combat would truly be the greatest, deepest game of this genre ever made.