I have been lucky to play with many GMs. With various degrees of experience and skill and enthusiasm. However, I do have an answer and I did not have to think too long about it.
This one goes back, WAY back. I had to be just out of high school when my cousin called me for a game one of his friend, a guy called Alex, whom I knew only tangentially was running a game of Rifts. He was a buddy of my cousin, in the days you gamed with the people you met through intermediaries.
I showed up and he was running what can best be described as "Rifts: Aliens". We had twelve people to start and over the course of two days whittled our numbers down to two. Me and "Sim" made it to the escape shuttle, only to realize that neither of us could pilot it. So Sim flew it out with his related Pilot: Airplane skill.
I have never been riveted to my seat before. His mastery of the GMing art was leaps and bounds above what I ever experienced before. Even though we knew what the adventure was, he played to the genre and really made the whole thing come alive.
It was a revelation in my life, and ever since them I wanted to be as good as he was. Bring in and atmosphere to the game that made the players feel like they were right there.