Best Playing/DMing Experience.
My best moment as a DM, I was running a World of Darkness campaign set in the setting of Innocents. Which is where the players are all characters under the age of 13. With World of Darkness the world is only slightly different than the real world. Think Supernatural.
With my very first session I started the players out in school. Gave them some bullies who were mean to them, parents, and teachers. Then the world changed. A virus killed all of the adults.
In my four hour session I described everything that happened over the course of the week where their parents died, I reduced my group of players to tears. They were all so into the story I was able to create a strong emotional response. Reducing a room to tears.
Honestly this has been the best compliment to my story telling abilities and I hope one day to be able to surpass it.