What was your most impactful RPG session?
My first interpretation of "impactful" is one wherein there was a session with great immersion or that altered by perceptions of RPGs in general. However, I have to say that no such thing has happened at my tables (though I have seen it at a LARP). When I play I always try to keep in mind that this is just a game, mostly so I can try and make it fun for everyone. Too often I have seen people ruin a good night for someone else because they are "in character". So, I try to remember the other players as we game which precludes getting too immersed.
However, there was one session where I was running a mono-language NPC in a Feng Shui game. Hugo was his name and that was the only word he spoke (long before Hodor and similar characters made it "popular"). I was able to convey multiple emotions and statements with that one word by use of inflection, tone and body language. He also became an NPC that the PCs actually cared about and went out of their way to help.