What was your most impactful RPG session?
This is actually pretty tough for me, because there are several issues around this question. Including the definition of "impactful", which I imagine is on purpose, to get the biggest response. The thing is, my table has always been a place for interesting things to happen to the player, rather than the characters. Sometimes interesting things happen to the characters, but that's not the point of playing -- the point is to make sure that the players are having a good time. So 'impactful', in my book, means moments when the players all sit up and gasp/cheer/react in some way.
So I don't know about the "most", but I can tell you about the "latest": when one of my players, a normally-quiet and careful player, decided that his character would take center stage, make a speech, and pull everyone into talking out of trouble, instead of fighting. Which, given we're playing RIFTS, was a pretty awesome and impressive for the player (and, consequently, the characters).
The goal, as a GM, is to give at least one memorable or "impactful" moment to everyone at the table at least once a session. Sometimes, I'm successful. Sometimes, not so much. But I think it's a reasonable goal.
What about you?