Immortal run in 25man Naxx, way before Ulduar patch. The run had gone smoothly up to that point. At last 12% Kel decides to MC my enhancement shaman and our retribution paladin and resets agro, heading straight for our squishy mage and all hell broke loose.
I went straight for our healer shaman with murderous intent. One nuke combo and she goes down to 20% before I'm finally cycloned. Ret paladin puts up a Hand of Sacrifice on Kel which in fact could kill him, thankfully our mage spellstealing it immediately. And that mage which is about to go splat? First hit takes him to below half life, second is a dodge and after that he had every friggin' cooldown on him that we had on our raid, giving our tank enough time to re-establish aggro.
The whole sequence left us breathless but yup, we got the kill and the achievment that we had prepared for quite some time.
Personally for me the best moment on any game or part of a game that makes me reach an actual "zone" where it's pure reflex to the extreme... But if I'd have to pick one, it'd be the final bossfight sequence on Zone of the Enders 2 on hardest difficulty.