Hmm, four moments vie for "best" moment for me. All from 3rd or 4th ed rules
The first was when I introduced one of my friends to the game. He had never played before, so I gave him the choice of playing either my Space Marine army or my Necron army. He chose the Space Marines, and after deployment I let him go first.
Movement stage was fairly standard, with explanations of difficult terrain and vehicle movement being made. Then came the shooting stage. My friend proceeds to prove he has the luck of the Emperor as his Landraider cuts down a Destroyer squad, his Devestators rip my Pariahs to pieces, and his Dreadnought blows up my Monolith. With a 6" explosion radius. Which claimed a squad of Warriors and a squad of Immortals. So I start my turn with 1 squad of Immortals, one squad of Warriors, one squad of Wraiths, my Lord and a single Pariah.
Wasn't impressed.
Second moment was playing my Necrons against a friends Alaitoc Rangers army, with an escalation deployment. For those who have never versed a Craftworld army, what with them not being generally legal anymore, the Rangers had a thing called the disruption table, which inflicted a variety of nasty results depending on a D6 roll. Look it up, it's quite interesting. Long story short, I started turn 1 with 3 Warriors on the field. And nothing else. Which, under the We'll Be Back rule, meant I had lost the game. Before turn 1 even began.
Wasn't impressed.
Third moment was playing a game with my Necrons at the local GW against a random Tyranid player. Turn 3 started with one of his Carnifex's wiping out my Wraith squad. Unfortunately for him, that left his Carnifex out in the open. My turn 3 melee round, I charged his Carnifex with my Nightbringer. Which won. His next melee he charged my Nightbringer with a half strength Genestealer squad. which failed to inflict a single wound and then died to my return attacks. My next melee I swept into his Hive Tyrant. And killed it. His next melee he jumped the Nightbringer with a Lictor. Which inflicted 1 wound before it died. My Next melee I swept into his Warrior squad, killing 2 and injuring the 3rd. His next melee, the rest of his warrior squad died.
He wasn't impressed.
Fourth moment was against a different Eldar playing friend. He had a squad of striking scorpions that made 39 armour saves in a single round of firing, and then proceeded to butcher my forces for the rest of the game.
Wasn't impressed.
C battle I've ever played in, it had everything; pincer attacks, alliances, betrayal, 1v1 uber unit fights, massive tank vs tank battles, raids and plenty of rebuilding our bases. 
