Civilization series - especially Civ IV is pretty awesome. Try the demo, I can almost guarantee you'll like it no matter what you think of the series right now.
Total War series - Rome is still one of the better games in the series but you might also want to check out the newest game, Shogun 2. I often hear people call it the best game in the series since Rome.
S.T.A.L.K.E.R. might do it. I'd say it mixes survival, FPS and RPG pretty nicely.
The first Crysis and its open levels are pretty fun. You won't find a great story there but at least you'll get a great game.
Enemy Territory: Quake Wars is not exclusive but the console versions are far inferior and you should definitely try Quake Wars. It's team-based action, a bit like Team Fortress 2 if you had no idea before. You can also check out its free predecessor, Wolfenstein: Enemy Territory.
Titan Quest is a nice Diablo clone. Or you can just stick to Diablo II if you wish, it's still very popular.
Also, some epic RPGs: Baldur's Gate series (especially BG2), Planescape: Torment, Fallout series (the older games), Arcanum, or if you prefer more combat and less story, why not the Icewind Dale series.
I'd also like to recommend Emperor: Rise of the Middle Kingdom. It's an older city-building game, set in China set in... Well, it begins when China began and ends in... I'm not sure, in the 17th century?
