What's the most underrated skill or trait that people should cultivate for a successful life?

Persistence

Before even I begin explaining, success is subjective. What I consider success might not be what success means to you.

But irrespective of that, what I have realised is that in life, you don't always need to be super creative or the most hard-working person ever to achieve the biggest of goals. All one has to do is to be persistent and consistent.

How often have we had that sudden burst of motivation to do that thing we always wanted to but that it faded away the next day? It's that complacency that's the biggest obstacle to success. And persistence is the only cure for it.

I tend to go back to 4 quotes which are also my guiding principles:-

Cowards never start, the weak quit along the way, and the one who remains wins.

Those at the top of the mountain did not fall there.

When I was practising, I became so exhausted that I wanted to give up. But then I pictured what my opponent would be doing. He must be still practicing so I went back into more practice. And a point came when I wanted to give up again. I pictured what my opponent would be doing. He must be still practicing so I went back into more practice. And then a point came when I could not lift myself anymore. And I pictured that he could not lift himself anymore either. I pictured him in the shower, and guess what I did. I went back into more practice. Till now we were at par. This practice would be my winning edge.

Its always the small things done over long periods that yield the biggest results.