Monster Hunter: Finally Killing Lao Shan Lung and Fatalis.
Halo Combat Evolved: Learning about the Flood, and the scene where Cortana and Guilty Spark argue.
Fear 2: Project Origin: The final cutscene, you know why.
Half Life: When you finally talk to the G-Man, it honestly scared the crap out of me and made the entire series worth it in my opinion.
Dead Space: Specifically finding out that I could get the Tank Suit free from the first store I found on Impossible. After that, it practically played itself.
Crysis: Finally getting through the door before the captain dies. The game glitched and it took me 4 hours working in the console to fix it.
Crysis Warhead: The airport battle.
Armored Core 4 Answer: Taking out the cradles and killing hundreds of thousands of people.
Armored Core Silent Line: Taking out the Satellite
Armored Core 3: Defeating the Controller.
Final Fantasy 7: The first time using Knights of Round
Myst 1-4: The endings were great imho
Starcraft: Kerrigan’s turning
Robot Arena 2: Design and Destroy: Creating a middleweight that could easily beat all of the heavyweights
Shattered Horizons: The first time I played it and MOVED.
Mass Effect 1 and 2: The stories
Fallout 3: First time using the Fat Man and the M.I.R.V.
Portal: Leaving the test chambers
Team Fortress 2: Holding of an invincible opposing team (10 of them) with 7 level 3 turrets, 9 engineers, and no losses to our side
Halo 2 PC: Realizing that even with a frame rate lower than 20, ridiculous lag, and the shivers from a very very cold room, I was still better than almost everyone online
Halo Custom Edition: Piloting a Scarab offensively in an online match
Halo 3: Finally killing Guilty Spark, and having him stay dead.
Pokemon R,B,Y: Capturing Mewtwo