Rome: Total War - Barbarian invasion - Taking on two full hordes with 6 Berserkers, 2 Lombard Archers and my General. It was ridiculous to watch and wasn't even a close battle. About half of my Berserkers were alive at the end. At that point I realized the Alemanni are the strongest faction in the game.
League of Legends - Ganking some poor sod before the first minute is up. Also, any time I get a 15+ kills, 0 deaths and 10+ assists, yelling "Om Nom Nom" and "Feed me some more!" into general chat. Which reminds me, I really should write that review of LoL that I was planning...
Dragon Age: Origins - Many times. Any Dragon kill was awesome, as were Ogre kills. Wiping out entire enemy formations with the Storm of the Century spell combo always brings a smile to my face. But my favourite moment was fighting Gaxkang (secret boss) and dropping 50% of his health with a single spell (experienced DA:O players probably know which one).
Assassin's Creed 2 - The whole game. Seriously, you play a Reneissance Italy Batman which is like regular Batman only with less emo angst and more banging hot chicks. But especially any assassination mission that I could execute with the Poison Blade without being detected. I just love sneaking by, finding the perfect route and then just swiftly poking my victim, then walking away like nothing happened.