For me, it was Wild Talents. @fray introduced me to it when we co-GMed a crazy convention game years ago. It was the first system that I felt had the freedom to really improv as a GM. If the players went down a different path it was no big deal for me to make up some bad guys on the spot since stats are so easy. I also fell in love with the chaos and group dynamics of the combat system. To me, it still feels like one of the most realistic fight simulators because of how the dice rolls create interaction between the PCs... for better or worse!
I think most of all, it was one of the first systems that I played that I could EASILY skin to be whatever I wanted. D&D and even Savage Worlds already seemed pre-flavored (I didn't know SW as well then as I do now!) but for whatever reason, when I learned WT it was the same feeling as when I see a pile of beautiful fabric. I might not know what it's going to become, but there is SO MUCH POTENTIAL!
It was a game changer to figure out I could learn the rules for one game and play ANYTHING with it.