Dragon Age Origins, because of the different origins giving multiple play-through a different feel
Mass Effect 2, the decisions you make/made in previous games can effect the game play experience greatly.
Fallout New Vegas has an absurd number of things to find/discover as well as the games decisions given alternate quest/story outcomes
The Total War series, you never know what the other nations will end up doing in reaction to you.
Super Mario 64, its just a really fun game.
This is the Game of Thrones
Where you either win
or you DIE