What is the most important non-technical skill for a successful career?

Kind of goes without saying, but communication. No one works by themselves any more, at some point in your day to day work life there'll be a time where you need to show something to someone else, and the quicker and simpler you make it, the happier everyone'll be!

I think hands down communication. But also the ability to get to know the environment quickly and mould yourself into that. I mean there can be introverts or extroverts and getting things done from folks in each case is totally different.

it may not be the most important but one underrated skill is pick the right problem to work on.
You can work super hard on something really difficult and come up with brillant solution on something that will have no impact whatsoever. And that can be an interesting or challenging experience but it won't really be beneficial in terms of career.
But by finding the right problem, scoping it right, making sure it addresses the right painpoints, and that you have means to really change things, adjusting it as you go, you can achieve tremendous results.
IMO there are lots of developers that focus on what they want to enjoy working on, and don't really question how they are positioned or if what they do is useful. Those 2 things are not incompatible, but from a career perspective it's infinitely better to work on the right thing and maybe use personal projects to feel the joy of coding.

I have written a blog post on 4 important non-technical skills that developers need to have to progress in their career.

nainacodes.com/blog/4-important-sk...

I think “listening another people” skill