If you really want an epic strategy game pick up Starcraft and it's exspansion Starcraft Brood War it is a little old but it's still a blast to play it in singleplayer you can also pick up Warcraft 3 Reign of Chaos and it's Exspansion Warcraft 3 The frozen Throne it's single player is not as strong as in Starcraft and Brood War but it is still solid.
And if you llike Turn Based Strategy be sure to pick up Civilization 4 and it's exspansion the game will give you hours of hours of gameplay and the feeling of really making a civilization.
If you like a mix of Turn based stategy and Real time strategy pick up Medivial 2 Total War and it's exspansion or Rome Total War and it's exspansion Rome is a little older and is set in the Roman Empire period while Medival 2 is set in the Middleage that's the main diffrence on those games and that the graphics on Medival 2 is a little better
Personal opinion: Starcraft the best game up there but it is also the oldest and it is really hard and fast paced but it also got great voice actors and a great story.
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