Could be a breathtaking moment like gazing upon Sovngarde's sky for the first time in skyrim, or maybe a scary moment in, uh, bioshock? A moment that stands out the most amongst all of your games.
Mine's probably in Oblivion. I had just started the game, and was going to this mountain (or hill) called Cloud Top, for a quest. When i reached the top, the climix of the atmosphere music (It was atmosphere_1, think its Glory of Cyrodiil) was playing, and my gaze rested upon the breathtaking view from the top of the mountain. I live in a city, so i don't get many tranquil views. The overview of the forest was outstanding. Not to mention the sky above; i felt immersed in paradise.
That was the moment for me