Mine was a pure roleplaying moment. Our group doesn't go heavily for roleplay but when we do dip in, it's usually intense. This one was born from a moment of stupidity, but it still the most intense I've felt from a game.
This time we infiltrating a mansion/compund of vampires, party around 11th. We had been dealing with these vampires on and for 3 months or so campaign time, since they controlled around half of the cities underworld. Sick of being manipulated and thoroughly outmanouvered, this was the partys' chance to hit back at the bloodsuckers.
In the back of the compund we had discovered a well and that was our entry point. It descended into darkness. My character (female human sun cleric) was first shock trooper down the rope, while the Ren, the rogue, spider climbed in but kept close.
Well the DM was amazing, describing in chilling detail the descent on the rope, the well walls dissapearing after 50' and the feeling of a larger cavern. 40' later, the cleric has hit the floor. The other players were just as awestruck by the intensity and imagery, the playing room was just as eerily silent.
So I whisper... "Ren?"
Whispers back from nearby "yeh?"
"I can't see."
<DM realises that neither can the human rogue, but doesn't miss a beat>
Whispers back "Uhh, me neither."
Still whispering: "OK, get ready"
I cast a Daylight and somehow the DM stepped up a gear, describing the intensity of the light and slow refocusing of my vision to see a field off coffins in every direction, and not 20' away, 4 vampire spawns caught in classic sneaking poses by the sudden light...
It was a real Aliens field of eggs kind of moment.
I was totally drawn in, the other players were silent, enjoying the spectacle, but I had toatally forgotten they were even there. Easily the best roleplaying moment ever for me.
Of course it was totally anticlimatic when exiting the very same chamber, my cleric died to a Slay Living trap when she needed only 4 or over on the save. Heh.