My 44th favorite gaming moment comes from the classic Descent 2. If you've not played the Descent series, take this fanboy's advice and pick up D2X-XL (which you'll need to play it on a modern PC) and buy the game (you can get it for practically pennies on ebay last time I checked).
Enough pitching. On with the list.
Descent had nothing in the way of cutscenes (at least not until the excellent Playstation port came out, with improved graphics and the cool "outrun the fireball" cutscenes between levels) and didn't really have anything for a definitive ending. Descent 2, meanwhile, ended up being a thrill for the eyes, with an actual full opening cutscene and cool briefing videos in between everything. It wasn't until the ending, though, that it really pulled out all the stops.
To this point, there hadn't been much story in the series except "kill robots, save hostages, blow everything else up". The idea that there might be a conspiracy injected into the action blew my frail little mind, and the idea of a sequel hook in a video game had been completely foreign to me at that point.
When Descent 3 was announced, I awaited it in a way that I had never anticipated a game before. To my horror, it was way beyond my primitive computer's abilities, and by the time I had a computer good enough to play it, the story didn't seem as compelling, and the controls seemed jumpy and annoying. The magic was lost.
Ah well, life is pain, I suppose.