I believe adaptability to change is the most important one as it comes with open-mindedness, responsibility, proactiveness, and positivity.

Empathy and kindness. I know that sounds cliche/fluffy, and I really am the furthest from someone who would throw out banal platitudes for no reason. So call it "Emotional Intelligence." But unless you're a one-person shop you're working with people, and even the most stoic coworker still has their own messiness and humanity. A group of people is more than just a bunch of individuals ticking off tasks, but only if they can figure out ways to work together well, day in day out, through all sorts of dynamics big and small, work related or not. And the thing is if you can master that (and, of course, be talented at your job): it's what people really remember.

Curiosity. There are those of us that are good at certain things, some of us never venture far from what that is, we do it throughout our careers with slight variations. For true growth, leave your comfort zone, be curious, ask what others do in your company, that may not have anything to do with what your role, contribution is. By understanding the other roles within an org, especially at the startup level, you learn how to better contribute in your role to perhaps set up your teammates better. Through curiosity you learn to evolve and adapt yourself to what needs to be done. Children are the best example of this as they learn when they're younger and they carry that with them until adulthood, but we do lose our level of curiosity. Stay curious, ask questions, jump into the unknown.

To believe in oneself is the soft skill which makes anyone a superhero :-)

Gotta throw a vote in for positive attitude - this helps so much with all the rest!

The ability to know when to speak and when to listen.

Being empathetic. Trends, technologies, companies, businesses, required skillsets change, but being able to understand and connect with people (users, customers, colleagues, teams, friends, etc) will always remain the basis of your success. IMO.

It's too hard to pick just one and arguably those individuals who are able to master multiple of the skills listed can absolutely transform teams. If I do have to narrow a list, I'd put adaptability and empathy at the top, especially in any early stage start-up.

Personal Branding

Some cultural anthropology and sociology...it helps us when we understand ourselves as humans, developing empathy and communication skills...and thus being more adaptable.

Growth mindset, positive attitude, and listening others

For carrer groth? The ability to speak your mind. It might make you a few enemies, but managers will always, always, always listen carefully to what you have to say.

They indicate longevity