I have already documented the experience that most changed the way that I play on this site here. The C Team shaped the way that I played forever after that game. But this question does not ask for the most heavy change, just an experience that made me change. Coming from my perspective – this will be about the way that it changed how I GM.
Dungeon World
My good friend Desiree Kaleopaa, who has written some articles on the blog, started gaming with Dungeon World. She would run it at her local library and kept telling me how good a game it was. I had no experience with it and so I joined in a game of it that my other good friend, Dan Hall, was running on Roll 20. It was my first game of it and it seemed pretty straightforward. We were running through a Dungeon and I was playing a thief. I saw a corpse with some jewelry and before you know it we are fighting a vampire for its necklace.

That game really shocked me. I was all worried because I was first level and it was a vampire. But we won. And the mechanics of this game were very interesting. Between this game and Desiree’s stories I bought the PDF and the soft cover and sat down to read.
Preparation Time
Being a GM has always come with the responsibility of preparation. This can be super fun but it can also be a huge grind at times. I like systems that cut down on preparation time. That is the beauty of a “World” game. Dungeon World, Apocalypse World and many others use the same philosophy and system. Learn as you play. This is equally true of the GM in these games. All you prepare is a skeleton of a world and then you run with the players and find out where their adventures take them! It is too hard to describe how simple this is in such a short post. Regardless of the details, this learn as you play approach to playing and being a GM has changed all of the games that I run. Preparation time has practically disappeared and fun is high.
Truly, if you have not looked at these games, please get a hold of one. Dungeon World is good but there is some confusing stuff in it at times. Apocalypse World is brilliant from cover to cover. Mind you, you have to be prepared for the adult content in Apocalypse World. Just choose a world and get reading. As a player or a GM I am certain that before long you will be glad that it changed your gaming too! Keep rolling…