Mass Effect 3. The final boss fight with Kai Leng. One time, all of the gameplay elements came together to produce one absolutely unforgettable moment.
I was in my element. I was an Infiltrator, using my speed and my invisibility to evade Kai Leng. We practically danced around the Illusive Man's control room as I stripped him of his barriers and his armor. And then, I happened to turn around at the right moment, right after we'd traded our last couple of insults.

There he was, doing one of those ridiculous ninja jumps, his head slowly coming into view as his body flipped forward.

There I was, Javelin at the ready, zoomed in on him. The right scope mods, which I had, slowed time down for the first few seconds you zoomed in, letting you place that shot perfectly.

So, I had time to savor the moment as his face and my crosshairs aligned. That one, fleeting moment when he saw me and realized that he was a dead man, and not his knives or his fancy karate tricks were going to get him out of this one. He'd finally met me in a fair fight, and I had thoroughly dominated him and his flunkies.
And I like to think that I saw his expression change as those thoughts went through his head. Denial? Resignation? Horror?

I pulled the trigger. And the thing about the Javelin is, the Javelin has this delay and a charging sound before it fires. Still under the influence of the time dilation, I had a moment to doubt. Would the dilation run out before the gun charged? Would I miss?

I didn't. And the final game mechanic that clicked into place was the headshot mechanic. In Mass Effect 3, when an enemy dies from a headshot, their braincase pops like a melon. And that too, I got to see in slow motion.

So, that was the greatest boss fight ever. It wasn't imagined by some cinematic director with Hollywood aspirations. It wasn't a cookie cutter scripted scene pulled out of a happy meal. It was a one-in-a-million fusion between my skills, my choices, and the same rules that governed the rest of the game's content. And I'm going to remember it long after...

Nah. It's a toss-up as to what I'll remember longer, the sheer glory of erasing Kai Leng's smug face from existence, or the crushing disappointment of realizing that 343i was going to phone in the fight against Didact with another QTE.