Truth: when a man lies he murders some part of the world.
Wait, that's the best quality in a knight. At least according to Merlin, anyway.
For a GM, there are two of them:
1. The ability to craft a first-rate story.
2. The humility to throw away that story when the players want to make something better of their own.
The best GM I ever had put together the most amazing story for a campaign background you've ever seen. When our group wanted to riff off of it and go in a different direction, he worked with us to make the campaign truly our own. That was a once in a lifetime game to be involved with.
Good gas mileage and a big trunk. I've had my GM for about 12 years now and she's as good as the day we first got her. She's older now and a bit rusty, but she can still take me & my players on a good ride.
The first piece of advice I always give GMs is "Know your players."
Being a good storyteller is great, but it's really not helping you if your players don't care about the cool story and only want intense combats. Being a good tactician is great too. But it doesn't mean a damn when your players mostly want to interact with the NPC's.
Priority One should always be to understand the sort of things that your players like the most and then finding the sweet spot where those combined desires meet what makes the game most fun for you as a GM.
I really love those times in the game when the GM describes a scene or action so well I can clearly imagine it.
One of my favorites: our players were carelessly barreling through a crowded city when we abruptly came to the 'edge' of the sky high land mote the city was on. His description of how we suddenly broke through the crowded buildings into the blinding light and the roaring wind, nearly tipping off the edge over the roiling sea below, had my stomach do a flip.
Hmm, tough one because I don't think it is one specific aspect - I also have a few DM's (play in multiple campaigns).
For me, I feel like I need to keep the group entertained, that's my #1 job if you will - they need to have fun and that requires of me multiple skills.
I'll name one of them here that I feel is very important, but often overlooked...
...The ability to maintain flowing conversations. NPC's know what they know (whatever that might be) and when a PC asks them questions and talks with them, there should not be a lot of "oh's" and "um's" - the NPC's reaction should not be halting or slow to react as I think it takes away from the suspension of disbelief (imho). Good, flowing dialog (to me) shows that NPC's have a life beyond what the PC's see day to day and this makes the world feel more real - players get more drawn in and their imaginations have an easier time of taking over. You can see this in the faces of your players (at least I can in mine) and it's great.
Now, there is a time and place for halting, jarring dialog if that is an aspect of said NPC for example - if they are slow, unintelligent maybe.
This may seem like an obvious thing (flowing dialog), but most DM's I play with could do a lot better.
I'm not perfect, but I'm pretty good imo - it takes a good bit of improvisation and quick thinking and it's something that I really work at.