The elder Scrolls V: Skyrim, This game has so many add ons to it, made by other gamers, they are really well done too.
Half Life - and most of the additions to it , very good series of games.
Metro 2033
all the Tombraider series , I really enjoyed these games over the years.
Deus Ex: Human Revolution and the missing link addition Rage
Portal and Portal 2, these are a different type of game, you really have to use your head and skills to beat these games.