The Mass Effect trilogy. One of the few games that's ever actually made me cry. I'd love to be able to experience that again the first time. The gameplay, the story, are good enough, but the world it built and the characters they introduced... Garrus, and Legion, and Mordin, and (most of) the rest, I don't think it'd be possible to forget them.
I need to show my age here and go with the King's Quest games (I-VII). I went through them so many times, it is just an interactive story at this point.
Blaster Master. The whole game is mind blowing. It's an even mix of sci-fi and fantasy. It would appeal to anyone who is into both The Legend Of Zelda and Star Wars.
Dragon Age Origins or Inquisition. I love both so much, to re-experience them with fresh eyes would be amazing.