Portal I and II. Puzzle games. Great plot, great voice actors. Lots of humor. Challenging. Nearly no violence. Superb aesthetics. Great soundtrack.
Portal 2 is my all time favourite game and the most fun I've had playing multiplayer.
All the aspects of the games gel together so wonderfully to make a really immersive experience. I just loved how the backstory was unveiled as you went through the game (especially Portal 2). Plus Stephen Merchant as Wheatley is iconic.