What is the MOST IMPORTANT skill for a GM to have?

[Xamnam]

The ability to read a table and respond to what it needs at a given time.

I really like this answer, because it covers two separate issues very neatly. Both on the interpersonal level, making sure people are engaged, having fun, not bored or tuning out, and then on a fictional level, delivering scenes that matter to the characters (and players), reflecting their interests and agency.

 

[Hand of Evil]

Keeping the game FUN.

 

[Fatal_Theory]

Being self-aware is super important. If you're asking yourself "Why aren't my players engaging with the content?" then maybe the content sucks. It doesn't mean you're a horrible person it just means they aren't having fun doing what they are doing. Just use that as a way to find gaps and get better.

 

[overgeeked]

The single most important skill is improvisation and a strong second is at least a rough eye for probabilities. The players are chaos incarnate, if you can’t improvise you can’t run a game. Since the players are chaos incarnate and you will have to improvise, you’d better have at least a rudimentary grasp of probabilities and how they correspond to the dice.