How can I decide? There are so many great ones to choose from.You have Skyrim, for its awesome graphics and intriguing storyline (though repetitive at some points with quests/caves); Diablo, for the sheer awesomeness of the game; and many more. Though I do suppose I can find one that I appreciate more.
The Mass Effect series. Great concept, good design, and the trilogy's end (beloved Mass Effect 3) made me sit there at my computer for a good whole hour staring at the computer screen wondering "what could I have done different?"
Because of these reasons, Mass Effect gets my vote every time.