If I had to pick just one, then I'd have to say hand reading. It's the most important skill, imo, and you can't succeed without it. Of course, without patience, discipline and selective aggression you can't do jack either. But of all of these, I defo think hand reading is the most important.
P.S. Oh, and emotional stability is probably up there too. I know of a few reeeeeeeaaally good players that have severe tilt issues, but they're still here because they are incredibly good and compensate with skill for their lack of mental endurance. But they would make their lives a lot easier if they just learned to control their emotions.
Yeah hand reading is another big one.. you can have all the tools but this and like in poker will be very hard for you, players will sense this as a weakness cause obviously yo are playing with the hands infront of you and capitalize on it
Emotional Stability.. another big one.. I'm sure what I'm about to say everyone goes through but I'm sure I've gone through the worst variance in the world to get where I am now, who knows maybe its a combination of that and me tilting to make it seem a whole lot worse. But I know I'm alot better than what my stats suggest but still stuck at micros due to tilt problems.
I think I'm finally learning to over come them, after a few MTT cashes and me then cashing out a large sum of it I encountered bad variance countless times of me being 4-1 favourite( or alot of times it was more than that) post or pre flop and continuously getting sucked out on, I think in 30 games it happened 25 in a row. But I endured it I know at times I through money away but I backed away before I did any long term damage
Now in the last 20 games I have had an ITM of 57% only flawed thing with this statement is my profit margin should of been at least 20$+ higher.. lots of 3rd and 2nd cashes which I AINT happy with, mind you I think with alot of the 3rds I didn't have much options when you are 1/3rd or even 1/5th of the Big stack heads up. something I'm working on as I feel with HE, while being solid I play scared money sometimes which is silly.
Which leads me to my next point .. SCARED MONEY
Perhaps this has been a problem of mine for some while it still is but its alot better than it was, but basicly if you are gonna go into every game with this mentality especially near the bubble in SNG you are gonna get run over.