Who needs therapy if you've got "impactful" RPGs, right?
Impactful is such a weird word. I've had lots of memorable sessions. Impactful seems like "something that changed you outside the game" and I'm not convinced that's why I play RPGs, so…
Man, I dunno.
Okay, here's one, maybe.
Dogs in the Vineyard. Our second serious fight. We'd already had one smaller altercation earlier in the game, so the players had seen how the conflicts worked, and more to the point they saw how FALLOUT worked.
Second conflict, things started getting heated, and someone pulled out a gun.
And the players – some experienced, pretty seen-it-all gamers – kind of pulled back from the table and were like "Whoa. Shit. Hang on a sec."
That game made guns fucking scary. They made them as goddamn dangerous as they are, you know?
(Best of all, the way fallout worked, you got fights where you didn't really know how fucked you were until the shooting stopped.)
Dogs remains the only game in which the players (and, by extension, the characters) reacted to someone pulling out a gun the way real people would, and I (obviously) remember that, to this day.