I don't know if this would be my number one, since I've played so many throughout my life, but Portal 2 is definitely up there. The only game I've played in recent memory to make my somewhat-nihilistic self feel that much emotion. Hearing GLaDOS's and the turrets' goddbye's was somewhat saddening, but overall I was happy that my long adventure was coming to an end at last - that I was finally free from GLaDOS and just as importantly that GLaDOS was free from me. We were both ready to put down our gloves and move on, and after one final goodbye from my longtime rival ( as well as those irritating but adorable turrets ) I was taking my freedom and leaving for good. And let's face it, GLaDOS giving you the Companion Cube back once you're out the door put a very nostalgic smile on your face. Don't lie, you know it's true.