Well, the first, of course. If I hadn't played that first session, I would never have continued playing RPGs.
But that's probably not the intent behind this question, so let's disregard the first session. In that case, then it is the first session of Dungeon World that I played. Not only was this a new-to-me group that has turned into a tightly-knit group of friends, it also exposed me to how the Apocalypse World Engine is supposed to work. This opened up new games and concepts to my gaming. The "make it your own" mentality of the AWE community has made me a better GM because it requires me to think of the things PCs do and how that relates to the world and the fiction.